Complex compartments

* Use only `div_compartment_integrator` for compartmentalization in
  `fvm_multicell`. The policy will later be moved to a backend
  policy class.
* For now, disable validation tests that test different division
  policies (see above).
* Tweak comments and remove redundant `using`, following comments
  on PR#54.

* Use divided compartments to determine FVM coefficients.
* Pick correct control volume in FVM from sgement position (avoids
  off-by-half error.)
* Add colour override functionality to tsplot: `--colour` option.
* Add const accessor for cell soma.
* Source formatting, comments in `math.hpp`
* Fix `range_view`: was using incorrectly named type trait.
* Add unit test for `range_view`.
* Allow points of discontinuity to be omitted from L-infinity norm
  calculations.
* Add `-d, --min-dt` option to `validate.exe` to control time
  step in validation convergence tests.
* Add validation test: confirm divided compartment policy does
  not effect results on simple frustrum dendrites.
* Change default max compartments on validation tests to 100
  (ad hoc observed convergence limit at dt circa 0.001 ms;
  finder spatial division would required much finer dt.)
* Make NEURON validation data generation scripts use CVODE by
  default, and with `secondorder=2` when non-zero `dt` is given.

Feature/divided compartments

Consolidate validation tests (issue #41)

* Make `algorithm::sum`, `algorithm::mean` more generic,
  allowing use with array types.
* Add `div_compartment` compartment representation, that
  holds geometric information for each half of a compartment
  that will then be used in calculating control volumes.
* Add three compartmentalisation schemes/policies that
  discretize a segment into `div_compartment` objects:
    * `div_compartment_by_ends` divides based only on the
      segment end points and radii.
    * `div_compartment_sampler` forms frusta by sampling
      the segment radius at each compartment boundary
    * `div_compartment_integrator` computes the compartment
      areas and volumes exactly by summing all frustra
      in the intersection of the segment and the compartmnet
      span.

* Simplify trace analysis and reporting code in
  `trace_analysis.hpp`
* Consolidate convergence test run procedures into
  new class `convergence_test_runner`.

Feature/new test model

More range functionality, unit tests.

* Add documentation of template parameters for `filter_iterator`.
* Document use of `uninitalized<F>` for holding functional objects
  in `filter_iterator` and `transform_iterator`

* Make `test_common_cells.hpp` and `ball_and_taper.py` agree.
* Add `ball_and_squiggle` model that has a tapering undulating
  profile.

* New `filter` view: lazily selects based on predicate.
* Generic `front` and `back` for sequences.
* New rangeutil STL wrappers `stable_sort_by`, `all_of`, `any_of`.
* Consolidate common utility unit testing structures into
  `tests/unit/common.hpp`

* Tests for `math::pi`, `math::lerp`, `math::area_frustrum`
  and `math::volume_frustrum`
* Fix `math:pi<long double>()`.

modcc: Support for units syntax within a state block.

Add "lib" to search prefixes for libtbb

Address deprecated use of 'symbol' warning.

Julia 0.5 deprecates use of `symbol` instead of
`Symbol`. This patch just substitutes the
correct call.

Julia 0.5 deprecates use of `symbol` instead of
`Symbol`. This patch just substitutes the
correct call.

* Fix quoting error in library search.
* Add "lib" to prefixes when system is "Linux".

Feature/cable validation

* Implement Rallpack1 validation test (with a workaround
  for inability to set membrane conductance).
* Fix bug in L≠1 case in PassiveCable.jl (this may still be
  wrong).
* Fix bug in peak delta computation in trace analysis when
  both traces have no local maxima.
* Gentle failure on missing `numeric_soma.json`
* Allow multiple `-s` selection operations for `tsplot`,
  acting disjunctively.

* Amend data directory path in validation tests.
* Enmodulate `hh_soma.jl`
* Add HH channel reference data generations script.
* Switch `validate_soma.cpp` to numeric reference data.
* Consolidate common code in `validate_ball_and_stick.cpp`
* Add (nearly) Rallpack1 validation test (see below).
* Gentle failure on absence of reference data in
  `validate_ball_and_stick.cpp`

Can't yet override mechanism default parameter values,
so the cable cell model added to `test_common_cells.hpp`
lets the default stand; validation script will have
to use the default membrane conductance rather than that
given by Rallpack1.

Address bug in fix for cell clone ctor.

* Add test for cell clone semantics.
* Fix `1cell::cell(clone_cell_t, const cell&)` for correct copy semantics.

Fixes Clang 3.7 failure to compile.

Add analytic simple cable solver.

* Use consistent scaling for y[1] scalar voltage in hh_soma.jl
* Also: add more reserved target names to CMakeLists.txt
  helper function.

* Move generation and data to top-level validation directory.
* Make BUILD_VALIDATION_DATA and VALIDATION_DATA_DIR cache vars.
* Add helper CMake functions for data generation.

Note `validation/ref/numeric/foo.sh` is just a placeholder.

* Align defaults with values used in most of the NEURON
  validation scripts.
* Use consistent 100 Ω·m bulk resistivity across both
  NEURON test models and basic validation cells.