WIP: Adds typedef for Volume in Geometry
Adds typedef for Volume in Geometry.
This breaks since OneDGrid geometry relies on
AxisAlignedCubeGeometry of dune-geometry, and UGGrid relyes on
MultiLinearGeometry as well hosted in dune-geometry. If this change is accepted, the
Volume type has to be implemented in these objects as well.
Hopefully, many of the downstream projects rely on
GeometryDefaultImplementation so that they don't have to change anything.