There are multiple reasons for this name change:

* The name Tuple conflicts with the boost::Tuple class, which as a
different interface and feature set. This gets confusing, especially
since VTK-m uses boost quite a bit.

* The use of this class is usually (although not always) as a
mathematical vector.

* The vtkm::Scalar and vtkm::Vector* classes are going to go away soon
to better support multiple base data type widths. Having this
abbriviated name will hopefully make the code a bit nicer when these
types have to be explicitly specified.

Also modified the implementation a bit to consolidate some of the code.