Fragment-level population analysis

[cjcscott]

Fragment-level population analysis is currently broken; see for instance running example 71-modify-hcore.py.

[ghb24]

There are two different poopulation analysis methods, one acting on embedding class, and one on the fragment. Fragment one broken (giving the population analysis from a single cluster, rather than exploiting ways to construct global 1RDMs) - this should be fixed at some point.