A unit test for a coupled bulk-shell model

Together with the new test, this commit also contains a number
of fixes for the SurfaceCosseratEnergy class.  This class was
not unit-tested previously, and unintentionally I messed it up
badly during my recent modernization work.

Finally, SurfaceCosseratEnergy had apparently never been tested
without dune-curvedgeometry, which is however only an optional
dependency.  The commit therefore also adds the necessary
conditionals.

Then problem then is that the derivative of the outer normal
of an intersection is not available from the standard grid interface.
If dune-curvedgeometry is not available, this derivative is
simply set to a zero matrix.  Depending on the grid manager
this may not actually be correct, though.