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.

We can now completely control the approximation order by a const int,
which is much nicer.

This data is used by the compute-disc-error program to measure discretization errors.

Because these two classes are for the most part identical.

Currently, this merged version involves some trickery, including some template
specializations and multiple inheritances.  I hope to get rid of that, eventually.

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.

... with values expected to be given in the parameter file.

Scaling the trust region is the key to success, because in general the midsurface
deformation and the rotations will have values in different ranges.

Unfortunately, this bases needs to be switched on and off using preprocessor
commands.  Hopefully I'll find a way to make that prettier in the future.

It was only in the dune-gfe module for historic reasons.

The inline expression in the parameter file that we used previously is
too short and limited.  In particular, we want to be able to use the
same expression for error measurements.  However, for that we need
a function with a derivative.

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That really was a hack: the build system should do it.

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