Auto-build docs using sphinx
This merge adds a gitlab-ci configuration that uses sphinx to build documentation pages for this package. The docs are built on every commit, but only published for git tags. The docs will render here:
https://docs.ligo.org/kipp.cannon/python-ligo-lw/
The build should be fairly self-sufficient, requiring little maintenance, as long as the setup.py install_requires list is kept up-to-date relative to the actual library dependencies.
In order to have this build cleanly, the following library changes were made: