This implementation allows to implement gridgeometry which share the same underlying BasicGridGeometry instance. This means only one bounding box tree, element map, index mapper. This is useful when different discretizations are constructed on the same grid view.
The "old" class BaseGridGeometry is now just a thin wrapper around the new BasicGridGeometry. The BasicGridGeometry is identical to the old BaseGridGeometry but is now intended for stand-alone usage in the main file. Discretization-scheme specific grid geometries still inherit from BaseGridGeometry.
This allows for two new features
- the basic grid geometry can now be shared between two or more discretization-scheme specific grid geometries on the same grid view
- the base grid geometry allows exchanging the actual implementation of the basic gridgeometry if this is desired to add some basic common shared features