Motivation

PyLops's solvers tend to lack stopping criteria or have one/two simple stopping criterion directly implemented as part of the while statement in the run method (with one of them always being the max number of iterations).

This PR aims to introduce additional stopping criteria. However instead of adding them directly to the while statement , those are implemented as special callbacks with a stop member.

For the moment, only one of such stopping criterion is implemented based on the ratio between the initial and current residual norm or total cost function (ResidualNormCallback) and is added to all solvers that have a cost internal member tracking the history of the residual norm or total cost function.

Definition of done

• Added new callback ResidualNormCallback and stopping auxiliary routine _callback_stop
• Included callback based stopping criteria to basic and sparsity solvers
• Added tests
• Added documentation