The `DynamicCellSet` class is now marked as deprecated (as is the header
that contains it), and all non-deprecated code is moved to its
`UnknownCellSet` replacement.
Also added a deprecation warning for the VariantArrayHandle.h header
file and deleted a couple inappropriate uses of it.
Generally, fields that have a WHOLE_MESH association might be valid even
if the structure of the mesh changes. Thus, it makes sense for filters
to pass this data pretty much all the time.
Also cleaned up some code and comments to make the relationship between
`MapFieldOntoOutput` and `DoMapField` more clear.
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