Matrix-Free Linear Operator

[yguclu]

See @e-moral-sanchez's PR: https://github.com/campospinto/psydac_dev/pull/7:

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Provide a MatrixFreeLinearOperator class which is a subclass of the abstract class LinearOperator.

The new class allows creating a general matrix-free linear operator. The constructor only requires the domain, codomain and a callable dot method.

Notes:

• The provided dot method may or may not take an out argument.
• A transpose_dot method may also be provided (mandatory to instantiate the transpose() linear operator).
• Unit tests have been added.

Additional changes:

• We stop passing the deprecated tol argument in calls to SciPy's minres and we use rtol instead, which requires SciPy >= 1.12. This fixes #419.
• We stop supporting Python 3.8 because of SciPy 1.12. Python 3.8 is close to end-of-life anyway, see https://devguide.python.org/versions.
• We also verify the initial convergence in the minres linear solver to avoid iterating when the initial solution is good enough (which may cause a division-by-zero error).
• We also revert the changes to files psydac/api/settings.py and pyproject.toml which were erroneously made in PR #320