Separate the density from the SurfaceCosseratEnergy

This makes for a cleaner design.  Also, the density is identical
to the one in nonplanarcosseratenergy.hh.  Eventually, we will
replace the density code there with the new density class
introduced here.

The new density CosseratShellDensity does not implement a particular
interface yet.  It is not clear to me yet how such an interface
would look like, exactly.