Merge branch 'matrix-loadbalance-to-more' into 'master'

Matrix loadbalance to more

There was a problem when having the global matrix and using load balance
to distribute it. This is fixed by the MR.

These changes should be cherrypicked to the release, too.

See merge request !28
(cherry picked from commit 1b544df3)

Signed-off-by: Steffen Müthing <muething@dune-project.org>