Add Spin Projection for TPSCI

[nmayhall-vt]

In the ASCI papers, they add determinants needed for getting pure spin states. I assume this is just applying S2 to the state and collecting non-zeros. We should try to do this - especially since we already have all the parts working.

[nmayhall-vt]

This needs some kind of iterative procedure as a single S2 application doesn't access the full set of configurations to spin complete