Add ATOMDB

[barrettp]

Add the ATOMDB spectral line equilibrium and non-equilibrium databases as a sub-packages.

[fjebaker]

Hi @barrettp is this equivalently the APEC and APED models, or did you have something else in mind?

[barrettp]

I'm thinking of something similar to the PyAtomDB module. I believe that AtomDB is needed for the APEC and APED models. This has been requested by several possible users at the HEAD meeting.