Split off IO from `GreensFunction` class

[janssenhenning]

Atm the GreensFunction class is very tied to the Fleur IO and the LS basis

It would be useful to have the ability to transform into different decompositions, e.g. from d Green's function to e2g/t2g blocks or from ls to j

The idea would be to make the GreensFunction class more generic instead of spins, there could be just the concept of blocks in the matrix, inspired by Greens functions in TRIQS) and either subclassing or completely separating it for Fleur