This is how the new dune-functions bases should be used.  Also, it is yet
another step towards merging CosseratEnergyLocalStiffness and MixedCosseratEnergy.

This model was given to me by Patrizio Neff in private communication.

The code is a prototype.  It appears to work, but it has not been tested very
thoroughly, and quite likely there are some bugs left.  Also, the code contains
the shape of the reference configuration hard-wired (a sphere).

This patch adds several more implementations of operator+ etc for FieldMatrices.
In particular, you can now combine matrices of adoubles (the 'active' type from
the ADOL-C automatic differentiation library) with regular double types.

These new operators are needed for the NonplanarCosseratShellEnergy class,
which is coming in the next patch.

The old version kept was too big, and kept a useless row of zeros around.
Removing this row should lead to a tiny increase in efficiency.  More importantly,
it gets us closer to the CosseratEnergyLocalStiffness class again, which
is scheduled to replace MixedCosseratEnergy eventually.

This leads to some simplification

This is an important step towards making the 'mixed' Cosserat code more like
the regular one.  The mid-term goal is to merge the two implementations, because
there is way too much code duplication here.

The DR variable (the derivative of the microrotation R wrt space) had dimensions 3x3x3.
This is two big in a shell context, because there are actually only two space dimensions.
Hence the correct size is 3x3xgridDim.

The results without this patch were not wrong, though, only a bit wasteful.

This makes 2nd-order orientation functions work again.

The implemented formula was actually for the projection of Stiefel manifolds, of which
the orthogonal matrices are only a special case.  For this special case, one term
drops out and doesn't have to be implemented.

This marks the last step of porting the 'cosserat-continuum' application away
from the dune-fufem bases.  One must not(!!!) mix dune-fufem bases with
dune-functions bases, as they use different dof orderings at least in the P2 case.

Because this object doesn't exist in dune-functions bases anymore.

[cleanup] Move the FD approximation of the GFE function gradient from the GFE function into the test

It is a part of the test harness, and should not clutter the implementation of the
GFE functions themselves.

Needed for projection-based finite elements.

This patch also adds a unit test.  So far this test passes.