Interface to get electron intergrlas and density matrices from external QC packages

[gabrielasd]

The main inputs required by EOMEE are the electron integrals and density matrices.

We should add some functions to get them from other quantum chemistry packages. For example, the electron integrals could come from Gaussian or PySCF mean-field computations and the RDMs (other than for HF wfn) from PySCF or PyCI.

GBasis, ChemTools and FanPy have wrapper modules that could be used as guides for these functions.

[PaulWAyers]

PySCF 2 has a nice interface it seems. @msricher already interfaced PySCF to gbasis as I recall.

[gabrielasd]

Is it this function? @PaulWAyers @msricher

[PaulWAyers]

Victor is doing PySCF link (with Farnaz) for ChemTools for Google Summer of Code. So there is a good hint there too.

[gabrielasd]

The aim here should be to make a wrap around an IOData instance, instead of the QC packages as is being done currently (commit 7411f88)

Once IOData is able to load the two-RDMs from PySCF/PyCI (see issue 281 in its repository) this change is possible.