NWChem Gradient Interface

[jwaldrop107]

Description
Adds a module to interface to NWChem for gradient calculations via the MolSSI stack, along with tests and related refactoring.

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[ryanmrichard]

@jwaldrop107 is this waiting on anything or is it ready to merge?

[jwaldrop107]

@ryanmrichard The NWChemEnergyAndGradientViaMolSSI module is a little hacked together at the moment. It's supposed to satisfy the EnergyNuclearGradientStdVectorD property type, but is appending the energy onto the end of the return value. This would be okay to merge if we removed that module, but I believe @kaka-zuumi was using that on his side.

[ryanmrichard]

@jwaldrop107 I see. Thanks for the clarification. @jlheflin that means I lied to you with the changes I suggested to StructureFinder; undo them. You want to be using the 'NWChem : SCF Gradient' module for the gradient and the 'NWChem : SCF' module for the energy (at present this will run the energy calculation twice, so don't explore with too large of systems).