Switch XML parser to pugixml; one less runtime dependency and one less blocker to WASM via emscripten.

- Correct `.gitmodules`
- Update all git submodules to latest released versions, except google-benchmark
- Have CHANGELOG
- Breaking changes since v0.7:
  - A change in API: `arbor.cable_cell` has the labels and decor arguments swapped. I.e.: `(tree, labels, decor)`
-> `(tree, decor, label)`. Labels are now optional.
  - Mechanism ABI version is bumped to 0.3.1. https://github.com/arbor-sim/arbor/pull/1884
  - Remove the `generate-catalogue` script.  `modcc` accepts now a list of NMODL files and is able to spit out a catalogue.cpp file
  - Rename spike detector -> threshold detector
  - Remove access to time `t` in NMODL.
- Major dependency version bumps:
  - GCC: 9 and up
  - CUDA: 11 and up
  - Clang: 10 and up

* Simplify scanning of SWC records.
* `parse_swc` now retains metadata read from SWC comments.
* `parse_swc` is given two modes, relaxed and strict. Strict mode enforces contiguous record numbering and rejects SWC that looks like it has a single-point soma.
* Subtype `swc_error` according to error mode.
* Remove `point` class and corresponding unit tests.
* Add a RAII stream formatting flag saver, used in SWC record output operator.
* Replace included 1-point soma SWC example with a regular SWC from NeuroMorpho.

* Replace TCLAP with tinyopt, remove TCLAP from `ext/`.
* Replace `sup/tinyopt.hpp` with copy of external tinyopt, update use of `tinyopt` in unit tests and examples to suit.
* Use tinyopt's 'smallopt' interface in `modcc`, `brunel` example.
* Update tinyopt interface in `single` example.
 Fix our attributions and add HBP acknowledgement

Addresses #557 #695 #803 

* Fix attribution for the Cephes library.

* Small language fix

Provides the following transcendentals intrinsics:

* `nmc_mm256_exp_pd`
* `nmc_mm256_log_pd`
* `nmc_mm256_pow_pd`
* `nmc_mm256_frexp_pd`

The first three are the equivalent of the corresponding SVML intrinsics without the `nmc` prefix.

The last one is used by the `log` function, but I decided it's nice to have it public.

All results are tested against full precision standard library implementation and provide equal results (except for the `pow` case). `NaN`s and infinities are treated according to the standard.

Limitations:

* Subnormals are treated as zeros by `frexp` and as a result by `log`.

Ben Cumming authored 7 years ago and Sam Yates committed 7 years ago

Adds support for building documentation with Sphinx from reStructuredText-formatted files in the `doc` subdirectory. Automatic building has been verified with ReadTheDocs.

* Add basic documentation to the `doc` path.
* Use a git submodule and associated CMake to pull in ReadTheDocs theme at configuration time.