Change ParticleDensity interface for setting mesh parameters

Previously, the mesh parameters used for the ParticleDensity filters had to be provided in the constructor. However, this is different than most other filters in VTK-m. It also can be more error prone as it is easy to mix up unnamed arguments.

Add methods to ParticleDensityBase to allow setting/getting parameters like Dimensions and Bounds to make it more clear what the values mean.