By removing the ability to have multiple CellSets in a DataSet
we can simplify the following things:
- Cell Fields now don't require a CellSet name when being constructed
- Filters don't need to manage what the active cellset is
To help provide a better time writing VTK-m filter this streamlines
the CreateResult API to provide a focused set of methods, based on
how CreateResult has been used by existing filters.
