Generic implementation of L2-norm calculation using generic L2-projection

With !1609 (merged) we get a generic l2-projection. In order to use it for interpolation between two arbitrary grids (arbitrary function spaces already works), we need to implement

• 2d-2d intersections (!1625 (merged))
• 3d-3d intersections (!2977 (merged))

That would be a great tool for convergence tests.