Should FieldResult have a resulting DataSet?

Right now, all the field filters return a FieldResult, which contains a vtkm::cont::Field class. However, in many cases this is an inconvenient form. You really want fields attached to a vtkm::cont::DataSet object so that they can be further processed.

It is a pain for users to have to get the Field and build a new DataSet on their own. It can also be confusing to new users. Also, it is easy to make mistakes (for example, getting the association wrong). It would be better if the filter built a DataSet containing the new field (along with the old fields).