Add support for Plug'n Play algorithm

[paquiteau]

This PR implements Plug'n Play Method, following the notation of ¹. Two new algorithms joins the ModOpt Family:

• PnP ADMM : using the ADMM algorithm and variable splitting with no constraint compared to ²
• PnP FBS (a simpler version of Forward Backward, where no acceleration is performed.)

Also, to ease the use of PnP ADMM I also wrote a convertor from Gadient operator to Proximal operator.

Footnotes

1. https://arxiv.org/pdf/1905.05406.pdf ↩

2. Add support for ADMM #263 ↩

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[paquiteau]

In my opinion this PR however good does not produce good results in our application (the hyperparameter tuning is rather picky.
Moreover the FB algorithm could simply be tweaked into a PnP algorithm (it's just a matter of giving a custom proximal operator, and disabling the fista iterative scheme).
Thus I will close this PR for now.