FISTA solver

[mrava87]

Add cls_sparsity and sparsity submodule to optimization starting with the implementation of ISTA and FISTA.

This solvers could be easily tested on a slightly modified version of our tutorial in poststack.py following this notebook: https://github.com/PyLops/pylops_notebooks/blob/master/developement/SeismicInversion-reflectivity.ipynb