Make Xdmf optional

This removes required dependencies for hdf5 and libxml2 by making Xdmf output optional. Xdmf has to be enabled explicitly by setting OGS_USE_XDMF=ON.

This should at least bypass some build errors for some people.

cc: @TobiasMeisel, @nagelt, @dominik-kern

Closes #3066 (closed).

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