Enhance filters API

This refactors filters API to streamline field selection and execute requests.

1. Added a new vtkm::filter::Filter base class for all vtkm filters. This class provides Execute method to request execution. This helps remove all Execute variants from subclasses.
2. Filter::Execute accepts a vtkm::cont::DataSet or vtkm::cont:MultiBlock.
3. Added vtkm::filter::FieldSelection class which can be used to select fields to pass/map to result dataset. A FieldSelection instance can be passed to Filter::Execute to pick which fields to pass/map.