Model-exporting-refactor

[IvarStefansson]

Proposed changes

Creates three new model exporting classes:

• Equation residuals
• Iterations (promoted from tutorial
• Fracture quantities (some new fields, some refactored from base DataSaving). This makes the PR BREAKING, since apertures and specific volumes are no longer exported by default.

Main questions:
Do the current models seem like valuable additions striking reasonable balance between convenience and cost (size of exported files)?
How do we want to test these classes?

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).
• 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

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.

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[jwboth]

Looks overall very good to me. I only have comments with potential improvements.