By making these helper classes ExecObject, they no longer need to have a
device to be constructed. This is important for future plans to make the
filters not determine a device adapter now that dispatchers no longer
require a device adapter.
This will make it easier to support integrators as an ExecObject.
One side effect is that the integrators and partical advection
are not templated by the type of the field.
Regardless of the type of the field, there is probably little reason to
compute particle advection with less than 64 bit floats to account for
accumulated errors. This will make it easier to use these classes.
This commit removes `vtkm::filter::Result`. All methods that used
`vtkm::filter::Result` simply change to use `vtkm::cont::Dataset` instead.
The utility API on `Result` that was used to add fields to the resulting
dataset is now available via `vtkm::filter::internal::CreateResult`.
Filters now support executing on a dataset to produce a result dataset
with automatic mapping of fields to the output dataset. Fields to map
can be selected using `vtkm::filter::FieldSelection` class, which provides
constructors to map all or no fields, along with a selection of fields.
This updates all tests to use the new filter API.
