GitLab

* Proof of concept matching current behaviour with Breathe

* Reorganise C++ API docs to work without autosummary

* Revert to Sphinx-native search

* Move remaining pip deps to conda

* Remove unnecessary lunrsearch templates

* Remove lunrsearch from cmake

* Tidy up layout of API docs

* Get code blocks to work with Breathe

Breathe doesn't seem to support the Doxygen <preformatted> tag.
It does support the <code> tag, but better yet it supports using
<verbatim> tags to embed rst into docstrings. This commit changes
all <pre> tags to use RST verbatim, and updates the Python
documentation machinery to support it too.

* Clarified some language

* Have doxygen exclude undocumented classes and have sphinx fail on warnings for C++ API docs

* List custom forces and integrators last

* Add breathe to documentation CI

* Typo

* Fix link to custom forces in extras.rst

* Have Breathe process only public classes

* Strip OpenMM:: prefix from rst files to avoid colons in links

* Remove existing private classes from EXCLUDE_SYMBOLS

* Add comment to C++ cmake describing why we promote warnings to errors

* Revise documentation build instructions for new dependencies

apparently SWIG typemaps maps Python's list but not set with std::set.
Instead, Python's `set` is mapped with `std::unordered_set`.

The original code of `swigInputBuilder.py` is kindly in a mess. Too much
lines in a single method `writeMethods` which causes debug becomes
difficult. Also there are a lot of warnings about
[PEP8](https://www.python.org/dev/peps/pep-0008/).

A refactoring in the future may be a good idea.

Swigged python docstrings now include documented return values and type
information or their arguments. They are generated in numpydoc format.
Furthermore, all of the Python app layer docstrings have been changed
to numpydoc format. The filterPythonFiles.py script which helps to
generate the Doxygen Python API docs has been updated to reflect these
changes.

cubic/quartic/pentic/etc. terms are NOT working for some reason.

Cleaned up Python API for XmlSerializer.  Also ensure that deserialized objects are returned as the correct class (see bug 1827)

Support for serialization of Integrator base class, and two derived classes: LangevinIntegrator and VerletIntegrator. Also added support for python serialization via a single deserializeIntegrator()

Use Doxygen to generate API docs for Python.  This involved lots of changes both to Python API generation and to Doxygen doc generation.