Zero-electron systems

[PaulWAyers]

The code crashes when one attempts to get local data for the hydrogen cation (and presumably other zero-electron systems). For any zero-electron system, all the local properties should return as simply zero.

Currently, if you try to do this you get something like KeyError: (1, 1)

[PaulWAyers]

The same should happen for zero-electron spin-densities (where the multiplicity = number of electrons). E.g., the Helium triplet, Hydrogen doublet, should have "0.0" for all local properties of the minority spin channel.