VTK-m style guidelines dictate that all classes part of the API should
be in their own header file. Thus, we have moved the result classes out
of the filter header files and into their own headers.

We have also renamed these clases to ResultField and ResultDataSet to
better match the class naming conventions of VTK-m.

Since we are moving around these result classes, we have also revamped
their structure. They are now in a hierarchy with a common ResultBase
class. Additionally, all result classes have a reference to a complete
DataSet that can be considered the output of the filter. This will make
it easier for users to consistently deal with filter results.

These changes cover issues #60 and #62.