These allow to map an entity to its geometry type. Depending on the
selected provider and grid view, this is encoded as GeometryType or
StaticGeometryType. There are several implementations:
* MixedGeometryTypeProvider will always act like a mixed element
  grid and simply forward the dyanamic GeometryType. You'd normally
  not want to use this one.
* AutoGeometryTypeProvider will use a StaticGeometryType if the grid
  satisfies hasSingleGeometryType(). Otherwise GeometryType is used.
  This is the default choise which should work always.
* SimplexGeometryTypeProvider and CubeGeometryTypeProvider will
  always provide the corresponding StaticGeometryType. This can be used
  to avoid the costly polymorphic interface if your grid supports mixed
  elements, but you know, that there are only simplicies or cubes.