Documentation and minor change in the Peaceman well model

[keileg]

Proposed changes

This PR introduce contains a slight modification of the Peaceman well model, and more importantly better documentation of that model. Moreover, the units of some constitutive laws were added.

Consider in particular:

1. For the specific volume in Peaceman, should we take the volume integral over the interface or the subdomain? It seems the former has the better chance of being correct for 1d-3d couplings, but I am not sure.
2. Please pay attention to the formatting in the documentation of units. I prefer this style, but if we want changes, it is best to do that now, and define whatever comes out of it as a template. Also, should we always document in SI units, or are derived units (Pa, N etc.) also okay?

Types of changes

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

• [x] 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).
• [x] 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.