Draft: Add MergeDataSet filter

As discussed in this MR, it might be useful to add a filter that can merge PartitionedDataSet into one dataset. Maybe there are several questions that need to be confirmed before implementation:

1 In the SliceMultiple filter, we assume all data sets in PartitionedDataSet have the same cell types. Are there any assumptions to the data sets that can be merged together in MergeDataSet filter?

2 In the SliceMultiple filter, we can copy the connectivity array of each data set into a merged array based on an offset and then fill the cellset by cellSet.Fill(numPoints, vtkm::CELL_SHAPE_TRIANGLE, 3, conn), How to properly merge cellset in this general MergeDataSet filter where each dataset in PartitionedDataSet may have different types of mesh.

3 What is the proper type for this filter (where should we put it under the filter directory)?

What do you think about these questions? Are there any other questions we should take care of? Thanks for the help! @kmorel @ollielo