Change notation for prox operators

From what I see, all proximal operators in this repo use `σ` for the proximal step. This notation is confusing, all solvers in `RegularizedOptimization.jl` call something like `prox!(args..., ν)` hence `σ` in this repo is in fact really `ν` which is defined as `1/σ` in the other repositories ... 

I will open a PR to change this notation.