Merge branch 'refactor_operators_and_products' of https://github.com/dune-community/dune-gdt into refactor_operators_and_products

Conflicts:
	dune/gdt/test/CMakeLists.txt

Merge branch 'refactor_operators_and_products' of https://github.com/dune-community/dune-gdt into refactor_operators_and_products

Conflicts:
	dune/gdt/operators/projections.hh
resolved to remote version

- add visualization for ProductSpaces
- make it easier to visualize time-dependent DiscreteFunctions by adding
an optional filename_suffix argument