Implementation of an external Solver

Description

Add external solver, which can

1. export system matrix and right hand side to disk
2. call an arbitrary external command (vi the 'terminal')
3. read in the solution provided by the external command

Testcases in Testsuite!7 (closed)

Check before assigning to a maintainer for review

• the history is clean
• code is well documented and understandable
• there is a testcase (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 stable tests pass
  • testsuite tests run
  • new tests are actually running (e.g. check if they appear on CDash)