Implement more general initial guess mechanism

[pbrehmer]

Enable implementation of an initial guess function in Greedy. It dispatches on the info tuple and on the arguments that were passed to the assemble call. This is important e.g. when using quantum number conserving DMRG, where specific initial states (states with defined QNs) are necessary. On this occasion, the info tuple will also be improved.

• closes More general initial guess mechanism #31
• closes Add more quantities to info during greedy assembly #38

[pbrehmer]

@mfherbst This should be relatively ready to merge (from my side). Any thoughts so far?

[pbrehmer]

I now removed the Function type annotation and fixed the indentation. I also happened to find an edge case in the HamiltonianCache constructor when I was experimenting today, which is now correctly handled.