support vtkh 3d slice

In vtkh, there is a filter that can process multiple slices (https://github.com/Alpine-DAV/ascent/blob/develop/src/libs/vtkh/filters/Slice.cpp#L475). This filter uses a worklet to compute the distance from each point to each plane; it also supports data sets with multiple domains. After processing each data domain, it uses a mergeCountour filter (https://github.com/Alpine-DAV/ascent/blob/develop/src/libs/vtkh/filters/Slice.cpp#L517) to merge data into one data set. I am wondering what is the proper strategy to support this filter in vtkm. Is it possible to use the multiplanes(!3124 (merged)) we added recently? Thanks for the help! @kmorel @ollielo