Add support for primal domain regularization

[Obi-Wan]

Description

Currently, we only support a few basic (mostly non-regularized) solvers, and the Primal-Dual Gybrid Gradient (PDHG) solver from Chambolle and Pock. The PDHG algorithm usually applies the regularization in the dual domain [1]. Other algorithms, like the Fast Iterative Shrinkage-Thresholding Algorithm (FISTA) from Beck and Teboulle apply the regularization in the primal domain [2]. The current code does not support them. In this PR, we want to introduce support for these algorithms and implement FISTA, as an example.

TODO

• Implement new feature:
  • Implement primal domain proximal applications
  • Implement FISTA
• Add or update unittests.
• Add or update docstrings.
• Update documentation.

Notes

[1] A. Chambolle and T. Pock, “A First-Order Primal-Dual Algorithm for Convex Problems with Applications to Imaging,” J. Math. Imaging Vis., vol. 40, no. 1, pp. 120–145, Dec. 2010, doi: 10.1007/s10851-010-0251-1.
[2] A. Beck and M. Teboulle, “A Fast Iterative Shrinkage-Thresholding Algorithm for Linear Inverse Problems,” SIAM J. Imaging Sci., vol. 2, pp. 183–202, 2009, doi: 10.1137/080716542.

[Obi-Wan]

This looks good now. I will merge, as this mostly introduces new functionality (few possibilities for regressions).