DataSet → DataSetProxy
Export nix::DataSet + make it base for DataArray

Need a wrapper clase since DataSet is abstract.
NB: This needs nix pr #414 merged due to DataSet::dataType()
DataSetProxy→DataSetMixin, DataArray is a DataSet
move ::data_extent prop from DataArray to DataSet
[test] Adap for DataArray is a DataSet change
Merge pull request #92 from gicmo/dataset

Export nix::DataSet and make it a base of DataArray