Implement different phonon frequencies for real phonons and the TFD bath phonons

[matthewcarbone]

This was a request by Stepan/Mona some time ago. It should be relatively simple to implement.

[Chiffafox]

Nice! Let me just remark here, for the record, as to the purpose of this. This feature would allow us to implement a routine for calculating mean phonon numbers in the polaron cloud -- both for the real and the fictitious sector. To do that, we need to be able to adjust frequencies individually, to compute the frequency derivative of the polaron energy.

Information about the cloud can help us understand the polaronic state better, as well as illuminating us as to the nature and meaning of the fictitious sector.