Draft: Feature/library naming

Closes #8 (closed)

As discussed in #8 (closed) I reworked our module structure to achieve a more unified project. The main changes are:

• move src/cpu/VirtualFluidsCore to src/cpu
• move src/gpu/VirtualFluids_GPU to src/gpu
• move other modules from src/cpu and src/gpu to src/

Because I moved so many files, the diff in gitlab take some time to appear. This is the new structure:

However, as a next step I want to archive the unmaintained libraries like Gks and demCoupling: https://git.rz.tu-bs.de/irmb/VirtualFluids_dev/-/issues/70