Attributes as classes: program-specific extensions

[berquist]

From the current version of the paper:

To revisit the ORCA example within v2, the etenergies instance still contains the current (uncorrected) excitation energies, but any additional sets of energies are also be present.

Notes: This is tricky because we don't know how to distinguish here yet. I'm thinking data, metadata, and extensions attributes. I will find the QCSchema slides that show an implementation of the latter. We could have some sort of "plugin" architecture here that also uses classes rather than just a loose dict, so replace SPIN ORBIT CORRECTED ABSORPTION SPECTRUM VIA TRANSITION VELOCITY DIPOLE MOMENTS with class TransitionEnergies and class TransitionMomentsVelocity and some tag that it's SOC corrected.