Add UncertainCellSet

The DynamicCellSet class has been replaced with UnknownCellSet. Likewise, the DynamicCellSetBase class (a templated version of DynamicCellSet) has been replaced with UncertainCellSet.

These changes principally follow the changes to the UnknownArrayHandle management class. The ArrayHandle version of a polymorphic manager has gone through several refinements from DynamicArrayHandle to VariantArrayHandle to its current form as UnknownArrayHandle. Throughout these improvements for ArrayHandle, the equivalent classes for CellSet have lagged behind. The CellSet version is decidedly simpler because CellSet itself is polymorphic, but there were definitely improvements to be had.

The biggest improvement was to remove the templating from the basic unknown cell set. The old DynamicArrayHandle was actually a type alias for DynamicArrayHandleBase<VTKM_DEFAULT_CELL_SET_LIST>. As VTKM_DEFAULT_CELL_SET_LIST tends to be pretty long, DynamicArrayHandle was actually a really long type. In contrast, UnknownArrayHandle is its own untemplated class and will show up in linker symbols as such.