Post-processing for MPSE, etc.

[jtv3]

Need to revive the post-processing routines for:

1. calculating the MPSE from a loss function, either provided by a valence calculation or another source
2. convoluting a valence or core calculation given an MPSE calc
3. Inputing the corrections pre-BSE (maybe?)

The first two at least should rely on some set API from the MPSE/Cumulant routines of @jjkas . Then we can modify the OCEAN routines to work with that.