Face error computation

[IvarStefansson]

Proposed changes

Improves the error computation for face values with a volume-like measure. Also introduces optional parameter for weighted norms. Testing of l2_error function is improved. Solves #1267

Types of changes

What types of changes does this PR introduce to PorePy? Put an x in the boxes that apply.

• [ ] Minor change (e.g., dependency bumps, broken links).
• [ ] Bugfix (non-breaking change which fixes an issue).
• [x] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [x] 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

Put an x in the boxes that apply or explain briefly why the box is not relevant.

• [ ] 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.