Implement cosserat-continuum.cc without CosseratEnergyLocalStiffness

Instead, use LocalIntegralEnergy and the newly introduced densities
for bulk models or planar shell models.

While doing so, give an error message when non-trivial volume loads
are requested.  I have no example file that actually uses them
at the moment, and just by looking at the code I find it hard to believe
that they actually worked.  Getting them to work will mean having
introduce a density for volume loads, which don't want to do
until I need them.

Since this is the last use of the CosseratEnergyLocalStiffness class,
this patch remove the entire class as well.