DataView: Export as nix.core.DataView
MulitTag: export retrieveData (as retrieve_data)
[test] MultiTag.retrieve_data tests
Move "data_type" prop from DataArray to DataSet
Move DataSet export logic into its own file

It was getting quite crowded in PyDataArray.cpp. Now there is
space in PyDataSet.cpp.
PyDataSet.arrayEnsureShapeAndCount: by-ref params

Actually wasn't doing anything before. Doh!
Merge pull request #93 from gicmo/retrieveData

Support for MultiTag.retrieve_data