Tpsa discretization class

[keileg]

Proposed changes

This PR introduces a two-point stress discretization for the elasticity problem. Tests are also provided.

An integration of the discretization method into the multiphysics models is in the works, but is not included in this PR. Also, a tutorial documenting usage may be included at some point, but for now, the documentation is in the form of a MVE inside the tpsa class.

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.

• [x] The documentation is up-to-date.
• [x] Static typing is included in the update.
• [x] This PR does not duplicate existing functionality.
• [x] 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.