This makes is slightly easier to use as you do not actually have to
construct the DataSetBuilder object but just call its static method.

The DataSetBuilderExplicitIterative methods are not static because they
use state of the builder object to create the data.

No need to use the device adapter, just do a manual host side copy.

Mainly issue dealing with dimensionality of cell sets and what that represents.
Have added in code to allow user to specify a custom dimensionality so that
tests continue to work properly.

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.