Merging this now since it's basically functional and we're about to split the repo.

Conflicts:
	.travis.yml

Resolved to remote version.

Conflicts:
	.travis.yml
	dune/stuff/test/grid_provider_cube.cc
	dune/stuff/test/grid_provider_dgf.cc
	dune/stuff/test/grid_provider_gmsh.cc

Resolved grid_provider files to local (deleted).
Resolved travis.yml to local except that it includes the new licence header now.

# 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/test/common_string.cc

Throwing and #error in this case generated a hard dependency
on istl or eigen. Therefore introdocue a default that'll
generate an error only if it's instantiated.