avoid deprecation warning

The method leafView has been renamed to leafGridView. However, the grid
check still called it, even with DISABLE_DEPRECATED_METHOD_CHECK. This
patch calls leafGridView instead.
add default implementation for lbegin / lend / leafbegin / leafend

Grids fully implementing their grid views (like GeometryGrid) just
forward calls to the mentioned methods to the corresponding grid views.
This patch migrates these implementations to GridDefaultImplementation
to prepare the deprecation of those methods.
properly deprecate methods leafView and levelView

Only the non-template versions of leafView and levelView have been
deprecated, yet. This patch also deprecates the template versions as
described in the documentation of <dune/common/deprecated.hh>.
avoid deprecation warning

The method levelView has been renamed to levelGridView. However, the grid
check still called it, even with DISABLE_DEPRECATED_METHOD_CHECK. This
patch calls levelGridView instead.