We use it all over the place. Adding a guard everywhere
would be insane.

mpihelper.hh is moved in dune-common >= 2.3

By now we make use of it in several places.

# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
#
# Lines starting with '#' will be ignored, and an empty message aborts
# the commit.

Conflicts:
	dune/stuff/grid/search.hh

Conflicts:
	dune/stuff/discretefunction/projection/heterogenous.hh

Necessary because no overloaded version of std::abs
exists for unsigned long long

Conflicts:
	cmake/DuneUtils.cmake
	dune/stuff/common/math.cc
	dune/stuff/grid/provider/gmsh.hh
	dune/stuff/test/fem_projection.cc