Add option to train to cohesive energies instead of total energies

[Efrem Braun]

I recently found out that when there are training images with different numbers of atoms present, training works much better if you subtract out the atomic energies from each of the structures such that you train to cohesive energies instead of total energies. I can do this with the existing Amp code by modifying the energies of the images before doing the Amp training, but it might be nice to have this be a feature in the code itself.