Previously WorkletInvokeFunctor inherited from vtkm::exec::FunctorBase,
which is also the base class for all users Worklets and for all functors
based to DeviceAdapter::Schedule.

This is done for a few reasons. The first is that we reduce the
minimum size of user worklets. Previously the users worklet would hold
a reference to the error message, and so would the wrapper class added
when calling DeviceAdapter::Schedule. Now we only have the users worklet
holding a reference.

Second, by refactoring to have two base classes we can better improve
the documentation on what responsibilities FunctorBase.h has, compared
to TaskBase.