Link SystemOperation to AbstractEnergySystem?

This is a question for the experts. So far we have the SystemOperation class tied to AbstractConversionSystem. It is (my guess) rather for historical reasons, as it was so before we changed the overall structure of the EnergySystem module in v. 0.9.

Now, would it not make more sense to move the SystemOperation class one step higher (hence linked to AbstractEnergySystem? I can imagine that also the other systems might have some operation schedules, not only the conversion ones.

@malhotra, what do you think?