Turn off redundant residual computations if tolerance np.inf

[jwboth]

Proposed changes

This is a minor change. Currently, the residual is always computed as part of the convergence check. However, in the case when the tolerance for the residual is np.inf, which is also the default value, this computation is redundant. This PR suggests turning off the unnecessary computation. This leads to some design choice how to forward empty residuals and norms. There may be other solutions. I am open for comments.

Types of changes

What types of changes does this PR introduce to PorePy?
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• Minor change (e.g., dependency bumps, broken links).
• Bugfix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• Testing (contribution related to testing of existing or new functionality).
• Documentation (contribution related to adding, improving, or fixing documentation).
• Maintenance (e.g., improve logic and performance, remove obsolete code).
• Other:

Checklist

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• The documentation is up-to-date.
• Static typing is included in the update.
• This PR does not duplicate existing functionality.
• The update is covered by the test suite (including tests added in the PR).
• If new skipped tests have been introduced in this PR, pytest was run with the --run-skipped flag.

[IvarStefansson]

I like the idea to avoid computation. Please consider the comment about optional arguments in particular.

[IvarStefansson]

With your clarification of Optional, the rest should be easy.

[jwboth]

Sorry for the massive delay...I fixed the remaining issues and followed all of your suggestions.

[keileg]

I think this should be fine now, but please have a short look as well @IvarStefansson.