Enable Python BCs in OGS Python wheels

Depends on !4318 (merged).

Many thanks to @bilke for assistance with library dependencies, testing and CI pipelines!

This MR enables the use of Python BCs and STs from OGS's Python wheels. This is a small step towards making OGS easy to use from Python, albeit a small step involving many changes.

What had to be changed?

• Ironically, not linking to Python libs in the Python wheel build
• Therefore, we cannot compile OGS as an executable in the wheel build. OGS will be used via @TomFischer's ogs.simulator module.
• Accordingly, the embedded OpenGeoSys module for BCs/STs does not exist. Python BCs/STs have to use the new ogs.callbacks module, which has the same functionality, as a drop-in replacement.
• The global project directory variable has to be cleared after each OGS simulation.
• The global matrix and vector provider has to be cleared rather than destroyed.
• The TCLAP command line parser must not exit the application but report the wish to exit (due to error or --help/--version flag) in another way.

Next steps

• Run the entire ctest test suite for the wheel builds.
• Getting rid of the global project directory variable.
• Making the ogs.simulator module more robust. Currently it's very easy to get segfaults if you forget to check the status codes of that modules' functions.
• Removing the global matrix and vector provider, thereby enabling multithreaded OGS invokations from Python 😵
• Add documentation to the user guide once the ogs.simulator module has been made more robust.

1. Feature description was added to the changelog
2. Tests covering your feature were added?