WorkletMapField: FieldIn/FieldOut. Template args + documentation

These don't take template parameters anymore, correct?

The documentation needs to be updated:

  /// \brief A control signature tag for input fields.
  ///
  /// This tag takes a template argument that is a type list tag that limits
  /// the possible value types in the array.
  ///
  struct FieldIn : vtkm::cont::arg::ControlSignatureTagBase