Increase flow velocities for partitioned tests

This MR increases the flow velocities for the tests of the partitioned coupling. This refers to the test case based on the FVCA test case and does replace the FVCA setting. We use larger pressure difference in the free-flow channel and have milder convergence measures for preCICE. This should ensure compatibility of the tests for preCICE v1 and preCICE v2. I used this opportunity to also add one test based on the Navier-Stokes equations.

• The new test resides in test/partitioned/flow-over-square-2d and the reference solution is residing in test/reference-solutions/partitioned/flow-over-square-2d.
• The pressure gradient is much larger (now 1e-2for Stokes and 1e-4 for Navier-Stokes vs. 1e-9 for the older case) than in the FVCA setting. This leads to larger flow velocities which should make the test case more reliable. Especially with the planned migration to preCICE v2 this should help to make the test case compatible for both preCICE versions. See #15 (closed)
• I added a script to easily update the reference data called create-reference-solution.sh that resides in test/partitioned/flow-over-square-2d.
• The Python script test/run-iterative-test.py driving the partitioned tests has been extende. It is now running the diff on the files reporting the coupliung iterations and the script is afterwards cleaning up the directory the test iwas running in.
• As a consequence of the changes to the Python script the CMakeLists.txt for the tests (FVCA) could be simplified
• I tried to replace most of the occurences of iterative by partitioned in the tests and test directories.
• This MR also changes parts of the CI:
  • The tests using the dumux-precice:3.3git-iterative-pr-1.6.1 image have been put to the nightly build are likely to fail this night. This image should be replaced/removed completely soon. A corresponding issue has been opened #19 (closed)