+ Slice splitting for diagonal parts of communication

Workaround for __BIGGEST_ALIGNMENT__ on Clang

See merge request walberla/walberla!458

On x86_64, this is always 128 bits, rendering it incompatible with AVX.
GCC reports the maximum enabled vector width (which is what we want) and Intel reports the maximum supported vector width (which is okay too).

Fixed possible divide by zero in mesh vtk writer and added case to test

See merge request walberla/walberla!455

However, since the macroscopic-value-backend is not yet implemented, it won't execute.