Merge branch 'bug/ug-messagebuffer-fails-for-aligned-types' into 'master'

ref:core/dune-grid We stumbled over a problem when trying to communicate
dense, fixed-size Eigen matrices:

The UG message buffer calls the copy assignment operator on all communicated
data to copy it into its internal void* buffer. This can fail for types that
assume alignment during copy operations, as there are no alignment guarantees
on the internal buffer.

The implementation only works for POD types anyway (as their raw memory
representation is communicated via MPI), so we just do the same when writing
to and reading from the buffer, which solves the problem.

See merge request [!300]

  [!300]: gitlab.dune-project.org/core/dune-grid/merge_requests/300