The `DynamicCellSet` (and the related `DynamicCellSetBase`) are
deprecated and replaced with `UnknownCellSet` (and `UncertainCellSet`).
Thus, `UnknownCellSet` has some methods inherited from `DynamicCellSet`
but replaced with other functionality. These methods are now marked as
deprecated and their use is removed.
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.