Implement CC for the unrestricted method

[aromanro]

Currently I'm implementing https://en.wikipedia.org/wiki/Coupled_cluster for the restricted method, hopefully that will succeed, but probably I'll postpone indefinitely the implementation for the unrestricted method.

I'll leave the issue opened for the future...

[aromanro]

CCSD(T) is implemented for the restricted method. It could be done for the unrestricted one as well.