Add analytic plasma demo using raysect's function framework

[jacklovell]

This is roughly 25x faster than the pure python, class-based demo.

Note that to build the docs for this requires installing the sphinx-tabs package from PyPI. sphinx and sphinx-rtd-theme are already additional requirements for building the docs: I wonder if we should define this somewhere (either in optional dependencies in setup.py or in a requirements-docs.txt).

[jacklovell]

Regarding the documentation dependencies, the pypa installer project has quite an elegant solution to this by providing a requirements.txt file in the doc folder, specifically for requirements when building documentation.

[jacklovell]

I decided in the end to go with an extras_require in setup.py: that way there doesn't need to be two separate pip invocations to install the package and the documentation dependencies.