Make units on energy inputs/outputs consistent, or at least document

[xorJane]

Right now the user specifies ecut and ecutprec in Rydberg. Energies included in the output such as <etotal> are given in eV, but units are not specified. Internally the code works with hartrees.

We should minimally tell the user explicitly that output energies are in eV. It may also be nice to allow the user to specify the units for inputs/outputs.