extend IegammaMVASvc interface to return energy without side effects

Presently the interface provides only the possibility to calibrate the energy of a particle, which means to change the energy of the particle. It would be useful to have a version without a side effect (i.e. without changing the cluster/particle), just returning the energy value.

This is particularly useful to be able to use the same tool used in reconstruction at analysis level (where systematics are applied), where this interface is expected.

Also, use double since that is the type returned by the underlying tool.

Tagging @christos