Move ibegin() and iend() from DefaultGridView into GridDefaultImplementation

Summary

The DefaultGridView is very useful to write grid wrappers easily. The issue is that in the current implementation we still have to implement wrappers for Entities and all what comes with that, since in the ibegin() methods an implementation method of the passed entity is called. Ineatd, this MR moves the ibegin() and iend() methods into the grid implementation as ilevelbegin(), ilevelend() and ileafbegin(), ileafend(). This allows to have a wrapper by just implementing the wrapping grid class and nothing else. This will simplify code a lot. Such a DefaultGridView is already used in some downstream modules, but it would be better to have this directly in dune-grid. Note, this change has no influence on any grid wrapper implementation if it is derived from GridDefaultImplementation where the corresponding methods are added as fallback implementation.