Add load & dump functions for atomic basis set definitions

[tovrstra]

This code was formerly present in HORTON2 in the module horton.gbasis.iobas. See https://github.com/theochem/horton/blob/master/horton/gbasis/iobas.py

The details should still be worked out:

• Should we store such data also in the IOData object, or in its own class? In the latter case, how can we avoid code redundancy?
• Which formats to support. Probably NWChem and Gaussian94 are sufficient?

Once this is in place, we it would require little code to construct molecular basis sets within IOData.

[PaulWAyers]

I think we need to support one format from EMSL, supporting two is good too, and NWChem and Gaussian are good choices. As long as we are compatible with a basis set from the EMSL we should be fine....

I don't see any reason to not store directly into the IOData object, but I defer that judgement to those better-qualified to make it.

[tovrstra]

I agree, it can go in IOData too. I'll make some amendments to #41.

[PaulWAyers]

gbasis already reads in NWChem format. So we could take that over to iodata and get at least NWChem format. @wilhadams can say more about how this works and how the JSON schema also has basis-set support.