Add functionality for excited states gradient and non-adiabatic couplings

[nathanshammah]

Potentially expand the functionality for excited states (see #105) to excited states gradient and non-adiabatic couplings in order to both go beyond ground state descriptions and move towards quantum dynamics. @bsenjean and @Akilan-QC feel free to add information, references and in general useful context.

[bsenjean]

Reference paper: https://arxiv.org/abs/2109.04576

[nathanshammah]

Removing from current milestone due to updates in priorities on the theory part.