It used to be in dune-functions originally, but got moved to
dune-common eventually.  This patch updates the code to reflect
that move.

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

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.

[[Imported from SVN: r10097]]

'Mixed' means that the spaces for deformations and microrotations can
be chosen separately.

At this point there hasn't been a lot of testing yet.  In particular,
parallelizing the assembly is currently not supported.

[[Imported from SVN: r9852]]