Adding in ZBL potential

[chrisiacovella]

Today we (@wiederm @MarshallYan ) discussed some issues that could be faced with free energy calculations related to overlapping atoms when energy is decoupled. As we have discussed in the past, adding in something like the ZBL potential to prevent overlap (even outside of the context of free energy calculations), could be useful, as the NNPs do not necessarily know anything about the energy landscape at really small separations.

this could be done in the post processing, similar to calculating Coulombic energy from partial charges. This just needs to know atomic numbers and the neighbor list information (pairs, d_ij). Even if two atoms are "lifted up" in a free energy calculation, we still can return a the value for the ZBL potential which would prevent overlap (i.e. not impacted by lambda).

I'll work on adding this in as this might be important for stability.