Allow Threshold::Run to work on dynamic cell sets

Added an overloaded version of vtkm::worklet::Threshold::Run that takes a dynamic cell set instead of a concrete cell set type. This is implemented by basically moving the CastAndCall from the Threshold filter to the worklet class. This allows other filters (that might have similar operations with different predicates) to use the same code. (It also happens to grately simplify an example for the User's Guide I am working on.)

Once cell sets use virtual methods, we should be able to collapse code like this quite a bit anyway.