I'd like to topologically erode the active voxels of mask grids (non level set grids) without changing their values or deleting off nodes. Unfortunately erodeVoxels calls pruneLevelSet by default. It also has no visibility on the grid classification (whether the tree represents a level set or not) so perhaps shouldn't be calling this anyway. Two issues:

• pruneLevelSet will always run, removing nodes if it's not desired
• erodeVoxels will fail if the background value is negative

We could expose a prune=true argument to disable the pruning, however still won't be able to call the correct prune method due to the lack of grid class information. Perhaps a new method is required, similar to dilateActiveValues.