Add preconditioning to CG solver

[VTrappler]

Hi,
I use pylops for solving large linear systems, and to test preconditioners.
Since pylops.optimization.solver.cg is very close to scipy.sparse.linalg.cg, I wondered if one could implement this feature, where the preconditioner is given as a linear operator which approximates the inverse of the original matrix (scipy's cg)
I have already implemented it as an new solver, if you are interested, I can prepare a PR.
Thank you

[mrava87]

Hello,
Indeed the two solvers should be identical. The only reason we implemented cg was to have a solver compatible with cupy arrays (scipy one is not). If you have this feature and can add it to our cg in a similar fashion to what scipy does, that would be great. Feel free to make a PR at any time :)

[VTrappler]

Great, I'll try to do it soon