Add `Implementation` type to `EntityIterator`

Ansgar Burchardtrequested to merge
bugfix/add-Implementation-type-to-EntityIterator into master

dune-fem was using this type previously made available by EntityPointer.

Note that the type is now always available as the DUNE_GRID_EXPERIMENTAL_GRID_EXTENSIONS has been deprecated.

See the discussion in !164 (merged)

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