Solver improvements

[dpo]

• Remove νInv everywhere
• Change verbose option so we log every verbose iterations
• Decide what to do with PG and Fista
• Add QN option when TRDH is used as subsolver (after update TRDH to use diagonal QN model #132)

[dpo]

Move PG, Fista and splitting solvers to https://github.com/JSO-Boneyard/nonsmooth-basic.

@MaxenceGollier