Fix issues with NumPy deprecations.

Several updates were made to align with the most recent NumPy changes (v2.0).

Specifically:

• numpy.float_ is now deprecated and numpy.float64 is used instead.
• numpy.infty is now deprecated and numpy.inf is used instead (there was mix usages).
• numpy.math is now deprecated. It was an alias to the math module in the standard library; For the relevant functions math is now used.

Moreover:

• the argument atol in the call to scipy.linalg.gmres() in the test function is removed due to deprecation and the default, whatever it is, is used instead.
• Clean up test files from commented block of codes and several new composite fixtures are introduced to simplify the code in the tests.

This commit should resolve Issue #147 (closed).