[WIP] Split SR into QGTMatrix and solver.
Created by: PhilipVinc
I put this here in case anybody wants to see the result of our discussion the other day. It implements the design described in #649 (closed).
I'm not done yet but shouldn't be too far off.
I think the only thing left to do for me is to rewrite the nk.optimizer.SR
in order to detect if using the old api/new api, convert to new api and print deprecation warnings. +tests