Cannot override energy spectrum for NEST sources

[robertsjames]

Peter Gaemers brought this issue to light - it's not currently possible to override the energy spectrum for a NEST source via e.g.

class myER(fd.nest.nestERSource): energies = tf.convert_to_tensor(blah) rates_vs_energy = tf.convert_to_tensor(blahblah)

This is related to how I've set the constructors up. Should be an okay fix, but need to rigorously test this to make sure all the logic with the Bayes bounds still works