If you have only a single cell shape, you can now efficiently create
a vtkm::cont::DataSet whose CellSet is vtkm::cont::CellSetSingleType.

Added cont/testing/ExplicitTestData.h which includes several explicit datasets.  These datasets come from VTK data generated in VisIt.  The new unit tests build datasets in several different ways and do some basic validation.

Add some new methods for DataSetFieldAdd class to improve usability.

Also, mark them as valid, when valid.