All the other math functions are in the vtkm package. This one was in
vtkm::exec because it uses a callback method. This can be problematic on
CUDA the the declaration of NewtonsMethod does not match the callback
method. However, we now have a VTKM_SUPPRESS_EXEC_WARNINGS macro that
allows a VTKM_EXEC_CONT_EXPORT function (like NewtonsMethod) to call
either a VTKM_EXEC_EXPORT or VTKM_CONT_EXPORT without a warning.
