Merge branch 'feature/FS1511-identitygrid'

This branch ports IdentityGrid to the copyable entities and
intersections, which is a little more involved than for a regular grid:

- All of the iterators / entities / intersections now simply store a
  copy of the corresponding host object - no more weird tricks with
  mutable objects, pointers to host objects in other IdentityGrid
  objects etc.

- The implementation of the EntityPointer is now a little more clever by
  copying the approach taken by GeometryGrid: Each iterator inherits
  from a different type of EntityPointer, but those are made
  interoperable with some casts. This makes it possible to store the
  host iterator for an IdentityGrid iterator inside the IdentityGrid
  iterator, avoiding the overhead of storing a separate host
  EntityPointer and iterator.

- The entity and intersection iterators now return temporary objects.

- As everything is now default-constructible, we can get rid of all the
  Makeable... wrappers

In total, the whole implementation has become a lot more
straightforward (apart from the performance trick for the iterators, but
even that is pretty easy to understand).

* feature/FS1511-identitygrid:
  [IdentityGrid][BugFix] Don't use deprecated typedefs for accessing host intersection iterators
  [test-identitygrid] Use a larger grid, scaled by dimension to keep grid size constant
  [IdentityGrid][11/11] Port to copyable entities and intersections
  [IdentityGrid][10/11] Port to copyable entities and intersections
  [IdentityGrid][9/11] Port to copyable entities and intersections
  [IdentityGrid][8/11] Port to copyable entities and intersections
  [IdentityGrid][7/11] Port to copyable entities and intersections
  [IdentityGrid][6/11] Port to copyable entities and intersections
  [IdentityGrid][5/11] Port to copyable entities and intersections
  [IdentityGrid][4/11] Port to copyable entities and intersections
  [IdentityGrid][3/11] Port to copyable entities and intersections
  [IdentityGrid][2/11] Port to copyable entities and intersections
  [IdentityGrid][1/11] Port to copyable entities and intersections

Conflicts:
	dune/grid/identitygrid/identitygridleveliterator.hh