Implemented LAMDA query

I've added a ``core.py'' and tests folder for the LAMDA database. Queries return Table instances for energy levels, radiative transitions, and collisional rates for all molecules and isotopologues currently listed on LAMDA. 

I haven't rigorously tested that each one works, but all methods pass for CO and return the right dtypes and column names. The special rovibrational files for H2O do not work because of a change in notation for the column headers.

Please let me know what else I can do before merging.