The ProductManifold class generalizes RigidBodyMotion, and can do
everything that the RigidBodyMotion class can.  Therefore there is
no point in keeping RigidBodyMotion any longer.  Having two
implementations for the same thing will just confuse people.

The old style has been deprecated for a long time, and is not
supported by dune-fufem anymore.

(In particular, the code discretizationerror.hh now requires
functions to satisfy the dune-functions function interface.)

In the wake of this, compute-disc-error.cc gets a number
of changes.  Let's hope I didn't screw up anything.

The code erroneously used std::tie to combine two local variables
to a single return value.  That may fail because std::tie produces
a tuple of references.  Once the method has returned these references
may or may not be valid anymore.  I am surprised that this bug
has not caused trouble much earlier.

Up until dune-common 2.7, the Dune namespace contained shared_ptr:
First as an actual implementation, later simply as a namespace
import.  Dune-common post-2.7 has removed that import,
and users of shared_ptr really have to explicitly specify
the std:: namespace prefix.

The original implementation measured the error of SO(3)-valued
functions in quaternion space in a few places.  This patch
converts another occurrence of this to matrix space.

'Only' a stupid typo: '=' instead of '+='.

In this case, two different refinements of the grids are not
necessarily geometrically identical.  This means we cannot
use hierarchic search with the global position of a quadrature
point, and expect to find the point on the other grid as well.

Instead, we go done to the coarsest grid, and make the transfer
to the other grid there.

Capturing any polymorphic object by value is wrong,
and gcc 8 started to warn about this in the case
of exceptions.

The latter is deprecated as of dune-functions 2.6.

Previously, the error norms where computed in the respective global
embedding space of the TargetSpace.  For the case of Rotation<3>
this was the space of quaternions, because this is how the
Rotation class is implemented.  However, this does not lead to
correct results.  Therefore, this patch adds special handling
for Rotation<3>, and computes the errors norms in the space of
3x3 matrices.

Make them a parameter instead.  This allows to switch more easily between
geodesic finite elements and projected finite elements.

It is still no function in the dune-functions sense, but at least the
basis parameter needs to be a dune-functions basis now, rather than
a deprecated dune-fufem basis.

For those, it is necessary to have separate measurement for the displacement and orientation components.

Also, check whether the input files have the correct length

This is a preparation to allow to select the TargetSpace at run-time.

Here is a new program that does nothing but compute discretization errors:
that is L2 and H1 norms of differences between two functions.  The reference
function can be either discrete or given in closed form (the latter is not
enabled yet).

So far I have only used this program to measure discretization errors of
Cosserat shell models, and I did *not* get the error behavior I expected.
So either the error behavior of such shells is weird, or this new measurement
program is still buggy.