HITRAN cleanup

There are several things that need to be improved in the HITRAN module:

* [ ] Docstrings need to show the accepted parameters in the callspec (I think we manage this in other packages?)
* [ ] Need to be able to query multiple molecules/isotopes simultaneously (can the database support these queries?)
* [ ] Need to add units to parsed tables

Start by locating the HITRAN module and its docstring, query, and parsed-table entry points, then compare them with similar packages in astroquery. Done means addressing the checklist: documenting accepted parameters, determining whether multi-molecule or multi-isotope queries are supported, and adding units to parsed tables.

Written by the indexing model from the issue text.