Add sphinx autodoc for LanguageModel

[johnjosephhorton]

We want the autodoc so the references thorough the documentation link back to this class:

file:///Users/john/tools/edsl/.temp/docs/language_models.html

[rbyh]

They already do?

Top of an .rst page should look like this:
.. _language_models:

And then references back to that page should look like this:
:ref:`language_models

[rbyh]

done