Added .gitignore to filter out non-dataviews notebook data
Initial commit of DataView Tutorial reference data
Added separate reference data folders for Python 2 and 3
Added reference data for the NdMapping Tutorial (py2 and py3)
Updated all reference data after fixing duplication bug

All reference data regenerated after fixing a bug in nbtest which often
duplicated output. Both notebooks have been checked to ensure that their
contents are still correct.
Updated reference data due to changes to Table and sample
Updated Python 3 reference data
Updated Tutorial reference data after restructuring dataviews
Updated reference data for all three tutorials (py2 and py3)
Updated Tutorial notebook reference data