MAINT: Deleted outdated todos

[keileg]

Mainly plain deletions, but I also expanded documentation at a few points.

Most of the deleted comments were fair and should have been fixed in a world with infinite resources, but chances are they would have gone unaddressed for a few more years if left alone.

Proposed changes

Contributions to PorePy are highly appreciated. Clearly explain why this pull request (PR) is needed and why it should be accepted. If this PR solves an issue, explain how it is done. Please, also summarise the changes to the code.

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.

[IvarStefansson]

Only minor comments. Feel free to resolve as you see fit.