... instead of the class CosseratEnergyLocalStiffness.  This is
one step in the quest to get rid of CosseratEnergyLocalStiffness
entirely.

Previously, it expected two factor spaces, as separate template
parameters.  Taking a single ProductManifold type is more consistent
with GeodesicFEAssembler, and it allows to eventually generalize
the code to more factors, if desired.

... but disallow a plain pointer.  This increases consistency with the
GeodesicFEAssembler class.

Rather than using preprocessor switches.  That's too fragile
and not extensible.

If the target manifold is a product space, then ProductManifold must be
used, and not a variadic list of the factor space types.

'thickness' and 'kappa' are parameters in Cosserat shell models.
They are not necessary for bulk models.  Hence remove them.

They do the same thing, essentially.

This is what the non-mixed ADOL-C assembler does, and I want to
merge the two.

Currently, Cosserat materials are hard-wired in the interface.
This patch changes the interface a little bit towards being
more general.

Change the cosseratcontinuumtest to check the newly implemented part in localintegralenergy and the bulkcosseratdensity