I think it wont hurt in other usecases

refs #191

This reverts commit 5d552579cc0467c80ca1d644ede91cd1fb56b371.

Conflicts:

	dune/stuff/function/expression.hh

Reverted because CMake hard fails if Eigen's not present:
CMake Error at cmake/FindEigen3.cmake:33 (file):
  file Internal CMake error when trying to open file:
  ABSPATH/multiscale-super/dune-stuff/../local/include/eigen3/Eigen/src/Core/util/Macros.h
  for reading.

can prolly be removed at some point