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.