Make divide by volume in particle density estimate a little safer
While going through the VTK-m code to identify where a cast-and-call was
happening against VTKM_DEFAULT_TYPE_LIST, I ran into a subtle case in
ParticleDensityBase
that was calling a worklet with an
UnknownArrayHandle
. This works OK, but was probably compiling for
unnecessary types (for example, vectors). Changed the field resolution
to be more intentional.