Mechanism Refactor: multicore and simd (#484)

First commit of two for mechanism refactor work (refer to PR #484 and PR #483).

FVM/mechanism code:
* Refactor mechanism data structures to decouple backend-specific implementations and mechanism metadata.
* Add mechanism catalogue for managing mechanism metadata and concrete implementation prototypes.
* Add fingerprint-checking to mechanism metadata and implementations to confirm they come from the same NMODL source (fingerprint is not yet computed, but tests are in place).
* Split FVM discretization work out from FVM integrator code.
* Use abstract base class over backend-templated FVM integrator class `fvm_lowered_cell_impl` to allow separate compilation of `mc_cell_group` and to remove the dummy backend code.
* Add a new FVM-specific scalar type `fvm_index_type` that is an alias for `int` to replace
`fvm_size_type` in fvm layouts and mechanisms. This was chosen as an alternative
to making `unsigned` versions of all our SIMD implementation classes.
* Extend `cable1d_neuron` global data to encompass: mechanism catalogue; default ion concentrations and charges; global temperature (only for Nernst); initial membrane potential.

Modcc:
* Collect printer sources in modcc under `printer/`.
* Move common functionality across printers into `printer/printerutil.{hpp,cpp}`.
* Add string to file I/O implemented in routines read_all and write_all in `io/bulkio.hpp`.
* Implement indent-friendly source code generation via a `std::streambuf` filter `io::prefixbuf` defined in `io/prefixbuf.hpp`, together with manipulators and a corresponding std::ostream-derived wrapper.
* Rewrite printers to use new infrastructure: cpu target incorporates SIMD printing options; CUDA printer at this point produces only stubs for CUDA kernel wrappers.
* Modify SIMD printing command line options for modcc: `-s` enables explicit vectorization using the SIMD classes;  `-S <N>` allows a specific data width to be prescribed.
* Fix problem in `test_ca.mod` with uninitialized ion current.
* Add infrastructure support to allow future pre-computation of SIMD index conflict cases for (hopefully) faster scatters and updates.
* Simplify `IndexedVariable` expressions in the AST, making data source explicit via a `sourceKind` enum, and leaving the indexing method and index names up to the printers.
* Allow state variables in the AST to 'shadow' an ion concentration — these are assigned in the
generated `write_ions` method.

SIMD classes:
* Add `simd_cast` operation between SIMD value types of the same width, and with `std::array`. (Note: this was tested and used in an early development version of the code, but not in this version. It was still a lacuna in the original SIMD wrappers, so it has been left in.)
* Restructure SIMD gather/scatter API to use a `simd::indirect` expression,  which encapsulates a pointer and SIMD offset.
* Add `simd::index_constraint` scoped enum to describe knowledge of contention in indirect indices, so that we can branch on this to the appropriate implementation.
* Add SIMD concrete implementation routines `reduce_add` for horizontal reduction and `element0` for access to first lane scalar value.
* Add SIMD value method `sum()` that exposes implementation `reduce_add`.
* Add SIMD concrete implementation routine `compound_indexed_add` that provides the implementation for `indirect(p, simd_indices) += simd_value` construction.
* Fix SIMD `implbase` bug where some static methods were using the `implbase` fall-back functions instead of the derived class specialized implementations.
* Move SIMD mathematical functions into friend routines of `simd_impl` in order to resolve implicit conversions from scalars in mixed SIMD-scalar operations.
* Use a templated `tag` class to dispatch on SIMD concrete implementation types, to avoid problems with incomplete types in method signatures.
* Remove old SIMD intrinsics.

CMake infrastructure:
* Downcase some variables in `CMakeLists.txt` files to  distinguish them visually from CMake keywords and variables.
* Split arbor modcc vectorization option (now `ARB_VECTORIZE`) and target-architecture optimization (now `ARB_ARCH`).
* For `arbor` and `arbormech` targets, and in particular not the `modcc` target, use `ARB_ARCH` to generate corresponding target-appropriate binaries, including, for example, appropriate SIMD support.
* Extend `CompilerOptions.cmake` to map as best as able between the various target architecture names (we use the gcc names) and the correct option to pass to the compiler based on the compiler and platform.
* Add work-around for misidentification by CMake of XL C as Clang.
* As a temporary work-around, include `arbormech` library twice on link line to resolve circular arbor–arbormech dependencies.

Unit tests:
* Extend repertoire of generic sequence equality/near equality testing support  in `common.hpp`.
* Add warning suppression for icc for the malloc instrumentation code.
* SIMD unit tests for indirect expressions, compound indirect add, reduction.
* Make some exact tests into floating point 'near' tests when comparing computed areas and lengths in swc and fvm layout tests, to account for compiler (e.g. icc) performing semantically inequivalent floating point operation reordering or fusion at `-O3`.
* Split out some of the CUDA tests into separate .cpp/.cu files for  separate-compilation purposes.

Other:
* The `padded_allocator` has been modified to propagate alignment/padding on move and copy (these semantics make their use much easier and safer in the multicore mechanism instantiation code).
* Map/table searching utilities in `util/maputil.hpp`.
* Fixes for correct sequence type categorization and `begin/end` ADL.
* Fixes for type guards for range methods that take universal references.
* Removal of some redundant code in range utilities through the use of universal references.
* Add new range view `reverse_view` for ranges delineated by bidirectional iterators.
* Add single argument form of `make_span` to count up from zero, and associated helper `count_along` that gives a span that indexes a supplied container.
* Moved `prefixbuf` to `modcc` source.
* Make sequence positive and negative tests in algorithms generic.
* Add `private`-subverting helper code/macro to `tests/unit/common.hpp` to reduce the number of public testing-only interfaces in the library code.
* Add virtual destructors for virtual base classes.
* Add new arb::math:: functions: `next_pow2` for unsigned integral types, `round_up` to round a number away from zero to next largest magnitude multiple.
* New `index_into` implementation that supports bidirectional access (moved to `util::` namespace).
* Fix problem in `test_ca.mod` with uninitialized ion current.
* Rework dangerous `memory::array(Iter, Iter)` constructor to be less dangerous (and do the expected thing).
* Allow ranges to be constructed from other ranges if the iterators are compatible.