Implement ProxSkip algorithm

[epapoutsellis]

Description

I strongly believe that this technique can actually improve algorithms developed for the last 20-30 years. Maybe a very bold statement but I stand by it because all my numerical experiments literally show this.

Check also Why-do-we-regularise-in-every-iteration

Environment

import cil, sys
print(cil.version.version, cil.version.commit_hash, sys.version, sys.platform)