Clarify variable names in forward_backward

[tkoskela]

The variable names between Algorithm 2 in Matt's Learned variational algorithms paper and the code it references don't match. It is making it difficult to update the algorithm. We should update the code to follow the naming convention in the paper in order to make changes to the algorithm to close #331.

[tkoskela]

The variable names in the code are a closer match to the Beck-Teboulle paper that describes the algorithm. Could still do with renaming p and out to what is used in the paper.