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.