Remove unit module; use scipy.constants

[msricher]

I removed the units module and used scipy.constants as per #17.

I also did unit conversions to atomic units in the NIST dataset @gabrielasd.

[gabrielasd]

Looks good to me. Thanks @msricher
These changes guarantee consistency in the unit format in which we store the data, and the conversions are taken from Scipy, which were the main requirements of the issue.

For the future, as enhancement, I'd suggest keeping the units module as a place to gather all conversion related variables. For example there could be a variable:
ev = constants.eV / (2 * constants.Rydberg * constants.h * constants.c)
like we have for amu and Angstrom.
Likely this should go together with cleaning up the compilation scripts like Michelle has pointed out, since in my opinion is something necessary for the sake of readability of the code.