Implement dedicated grid view classes for UGGrid

These currently simply forward everything to UGGrid itself (just like
the DefaultGridViews do).  But at least with this patch the infrastructure
is there to start moving the grid-view functionality from UGGrid to the
GridView classes.
Use entityPointer->level() instead of entityPointer.level()
[bugfix] Fix copy'n'paste error: f->level() instead of f->level()
Test EntityPointer::level only if DISABLE_DEPRECATED_METHOD_CHECK is not set
Deprecate the method 'level' on the EntityPointer and EntityIterator classes

In fact, the patch really only deprecates the method on EntityPointer.
However since EntityIterator inherits it, the deprecation also concerns
calling 'level' on EntityIterator.