This fixes #1

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

Fix method 'difference'

See merge request !1

The formula for the FD gradient never took the distance between
the two Lagrange points into account, even though the gradient
of an interpolation function clearly depends on it.

(cherry picked from commit 7f326ded)

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.

So far: 2d only

This will be used in the paper with Hanne, Philipp and Markus
on projection-based FE.

I want the interpolation rule to appear in the log files.
It is to easy to use the wrong one by accident.

Otherwise certain matrix-copying methods will not work anymore.

In particular, this patch removes tests for the gradient
of the Cosserat energy and related things.  The
CosseratEnergyLocalStiffness class stopped computing
these things back in 2014 (in 63e42ece )
when it was decided that algorithmic differentiation was
the way to go.

With this patch, the test builds again, but it doesn't run yet.
I am afraid that fixing that may require a larger investment
of time...

- 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.