Backport [!395] Make sure that parallel vectors are consistent after...

Backport [!395 (merged)] Make sure that parallel vectors are consistent after interpolating boundary conditions

Merge branch 'bugfix/make-interpolation-in-onestepgridoperator-consistent' into 'master'

ref:pdelab/dune-pdelab The interpolate() function on the one step gridoperator does make sure to preserve unconstrained vector entries across the reinterpolation of constrained DOFs, but this does not work correctly for constrained overlap values, as those are constrained on some ranks, but not on others.

This patch fixes the problem by broadcasting overlap data from the rank which contains it as interior data to the overlap regions.

This is a backport of !395 (merged).

See merge request pdelab/dune-pdelab!395