add fb algorithm, pnp-prior functionality, abstract classes, and kera…

[CosmoMatt]

Overview This PR includes quite a few bits and pieces that have been outstanding. Primarily it adds forward backward solver and enhances the codebase by supporting keras/tensorflow likelihood/priors.

Issues:

closes #5 closes #6

Additions:

• adds a basic forward backward solver (this has been tested externally and can function with keras priors/gradients)
• adds abstract classes to linear operators, proximal operators, ai operators, gradient operators.
• adds forward backward 1d notebook
• updates custom operator notebook to inherit the corresponding abstract class

[tobias-liaudat]

Hi @CosmoMatt and @MatthijsMars I was checking out this PR, is there a reason it has not been merged?