fixes make headercheck
[grid] make stuff/tests compile again after recent grid provider storage change
Merge branch 'master' of https://github.com/wwu-numerik/dune-stuff

Conflicts:
	dune/stuff/fem/functions/magnitude.hh
	dune/stuff/fem/matrix_object.hh
	dune/stuff/functions/femadapter.hh
	dune/stuff/playground/functions/fixed.hh
	dune/stuff/playground/functions/time.hh
	dune/stuff/playground/grid/provider/starcd.hh
	dune/stuff/test/grid_information.cc
	dune/stuff/test/la_container.cc
	dune/stuff/test/la_container.hh
[parameter.validation] fix typo
[common.parameter.configcontainer] add documentation
[common.parameter.configcontainer] change get/printMismatchedDefaults
- changed getMismatchedDefaults to return empty set if there
  are no mismatched defaults, as the name implies
- changed printMismatchedDefaults to print only keys that have mismatched
  defaults
[test.common_parameter_configcontainer] explicitly cast to avoid char issue