Deprecate DataSetFieldAdd?

(Low priority discussion question.)

Reading through the docs, it's not clear to me why we need an independent class DataSetFieldAdd.

It seems like we could just as easily have an AddPointField and AddCellField defined on the vtkm::cont::DataSet class:

vtkm::cont::DataSet dataSet;
std::vector<vtkm::FloatDefault> v;
dataSet.AddPointField("name", v);

Looking at the implementation of DataSetFieldAdd, it is just a wrapper around dataSet.AddField(make_Field(...)) and hence might be more ergonomic to just have it directly as part of the vtkm::cont::DataSet class.