Restructure repository and tests

This MR does a restructures the repository. This has the following goals:

• Be closer to the directory structure of dumux and other DUNE modules. We separate the adapter stricter from the tests and examples.
• Remove

The following changes have been made:

• The actual preCICE adapter code moved into the dumux-precice directory.

• The adapter was renamed from PreciceAdapter to CouplingAdapter which is more in line with the CouplingManager of the monolithic coupling. The source files have been renamed accordingly to couplingadapter.hh and couplingadapter.cc.

• The namespace has been renamed from precice_adapter to Dumux::Precice.

• The preCICE adapter is build as static library libdumux-precice.a. Using a static library makes sure that the executables are "self-container" and do not rely on the adapter code that is build intermediately for convenience. It does not have to be rebuild for each executable. The header is installed in the "include" path. This means that the header can be included as

  #include <dumux-precice/couplingadapter.hh>

• The current test cases have been adapted to respect the new file and class names.

• Moved FVCA test cases to test/ subdirectory. This includes the renaming of the directories and the executables. Test executables are prefixed with test_.

• Example cases moved from appl/ into examples/ directory.

• Moved additional monolithic tests (ff-pm-3d and flow-over-step-2d) into test/monolithic directory.

• Moved conjugated heat transfer into examples/legacy/cht as it is an older test case that is not working at the moment.

• Removed broken or incomplete cases from example/ directory. Most of these cases did not work and it is/was also not clear anymore what they should do. These test cases went into a private repository in case on would need them later again.

• The CI has been adapted to run only on the simpler test cases (flow-over-square-2d for partitioned and monolithic coupling) for commits. During night the full pipeline would run.