Reformat forces
FiniteVolume: Change In_Plane to in_plane
StrainMeasure doesn't need to be templatized over the ambient dimension

The constructor now takes rest coordinates as RawArray<const T,2>, so
that the ambient dimension is runtime adjustible.  Since we perfect a QR
decomposition when forming Dm_inverse, this ambient dimension is
immediately forgotten.
SimpleShell: Add a version of finite elements for simple anisotropic cloth