Add custom CommonDenseVectorBackend

This PR introduces (among others) a custom backend for the CommonDenseVector. The advantage is that now the CommonDenseVector can also be created using a pointer to a preallocated buffer. I use this to avoid copying the reduced basis vectors to all MPI ranks (which would need too much memory). I thought it might also be of interest in other cases, as it makes it easy to transform every buffer with continuous memory into a CommonDenseVector. On the downside, it adds complexity, so I can also remove that commit from this PR and just keep it in my HAPOD branch. Thoughts, @r_milk01, @falbr_01?