The interface of Solvers::IterativeSolver used to make its
iterationStep_ member publicly available.  This has changed,
and the iteration step can now only be accessed by a getter
method (which is good).  However, that getter returns a
reference, but not the full smart pointer.

See the discussion at

  https://git.imp.fu-berlin.de/agnumpde/dune-solvers/merge_requests/23

In theory this is dangerous, because a pure Newton method will
only converge locally.  However, the trust-region part had been
partially disabled anyway, and I have not noticed any problems.
Also, I think that from my well-posedness result for higher-order
GeoFEs follows that a Newton method will always converge if the
problem is well-posed (that is how the proof works, after all).

On the plus side, this patch brings roughly 5% speed increas.

These Hessians are always symmetric, and we should really
exploit this.

Additionally, it seems that the method was never used anyway.

This is necessary to compute H1 errors in matrix space.

This used to work, because the matrix is 1x1 anyway (with corresponding
vectors), but some time ago the matrix and vector assignment
in dune-common was reworked, and now the code does not compile
anymore.

This got introduced in dune-grid 2.6.

In particular, MonotoneMultiGridStep use shared_ptr much more
frequently now.  Also, various of its data members are now
protected, and getters/setters have to be used.

It used to be in dune-functions originally, but got moved to
dune-common eventually.  This patch updates the code to reflect
that move.

I think they used to have one, back in the day of the EntityIterator
class.

Transformed the surrounding loop to range-based-for syntax
while I am there.

The one in the Arithmetic namespace does not exist anymore.

And some other Dune 2.4 stuff.

It has been removed from dune-functions in the last release
(at least).

Previously, the actual projection happened by calling the constructor
of one of the classes that implements points on a manifold.  This patch
introduces an new method projectOnto, which makes the projection more
explicit.

- use more shared_ptr
- New MCMGMapper interface from dune-grid 2.6
- basis.indexSet() has been removed from dune-functions

Makes it faster, but only by 1 or 2 percent.