Draft: Thin layer formulation for vector field (Mechanics)

Description

https://gitlab.tuwien.ac.at/e325/03/thin-layer-formulation
Follow up for !126 (merged)

Testsuite merge request: Testsuite!90 (closed)

Resolves #177 (closed)

Check before assigning to a maintainer for review

• The history is clean
• New Code is well-documented and understandable
• There is a test case (in case of new functionality)
• Every commit passes the pipeline

Maintainer checks before merge

• Review is approved, and all comments are resolved
• Check for Testsuite changes, if yes check
  • Testsuite merge request merges fast-forward
  • Testsuite-submodule SHA of every CFS commit references a commit in Testsuite Merge Request
  • Testsuite-submodule SHA of last CFS commit points to HEAD of corresponding Testsuite branch
• Pipeline passes for every commit
  • all tests of the stage test pass
  • tests in the stage test_extra are created
  • new tests are actually running (e.g. check if they appear on CDash)