Merge branch 'feature/raviart-thomas-space' into 'master'
add RaviartThomasSpace
See merge request !212 (merged)
(cherry picked from commit 75f00bc9)
7f68756d add RaviartThomasSpace
8a2305b1 add alternative local interpolation for RaviartThomasSpace
da0e16a7 [raviartthomas] make sure only valid spaces can be constructed