Additional tensorial properties: pair with QCEl#241

[bgpeyton]

This is the sister-issue to QCEl#241. If the properties are to be added to the schema, it looks like we should solicit opinions from this repo as well (if I read #68 correctly).

We're looking to add some tensorial properties, namely polarizabilities and other (linear) response tensors such as the Rosenfeld mixed magnetic- and electric-dipole tensor. This is somewhat related to #70 in that these tensors are often redundant, but also unique in that the details of the perturbing field (order, operator(s), frequency) should also be considered. The openrsp model is promising, but perhaps more complicated than we need (for now). Any feedback is appreciated.

[ghutchis]

Yes, polarizabilities and quadrupole moments would be useful for a range of purposes.

[ringholm]

@robertodr told me about this issue. I am with OpenRSP and I am happy to answer any questions you may have about how we approached this.