write or use xyz writer that doesn't affect units

[joshkamm]

Once ZimmermanGroup/pyGSM#65 is fixed, update

py-conformational-sampling/src/conformational_sampling/analyze.py

Line 119 in 46c708e

self.string_energies.append(float(mol_block.split()[0]) / KJ_MOL_TO_AU)

to no longer need a weird unit conversion.
Since we already have results in the old weird units of AU*kcal/kJ, write some sort of logic to differentiate which version of pyGSM the results were generated with and handle the units accordingly.