Remove domain midpoint, radius, and convexity

UG used to store the midpoint and radius of a circle that contained the domain. This was mainly used by the graphics subsystem, which is gone now. Therefore we can remove the midPoint, radius, and convexity data members, and make the code a tiny bit simpler.

The patch changes the interface of the CreateDomain method, which is used in dune-grid. Therefore, dune-grid needs to be updated in lock-step with this change. I will post the corresponding patch shortly.

mentioned in commit core/dune-grid@7e17d30a

mentioned in merge request core/dune-grid!146 (merged)

mentioned in commit e0e37c6b

merged