L2Norm: openMP-parallelize and add option to compute only last time step ...

Description

• Add an option to compute only the last timestep as usually for transient simulations it is not necessary to compare all timesteps to an analytical solution.
• Add openMP parallelization for the iteration over elements to speed up the computation

Testsuite merge request: Testsuite!72 (merged)

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 run
  • new tests are actually running (e.g. check if they appear on CDash)