[test] DataArray: modify IO tests to prepare for 'data' change

With 'data' the DataArray.data property is meant
PyDataArray: Remove data property and its docstrings
DataArray: re-add 'data' property which now returns a DataSet

Or it returns None if there is no data associated with the object.
[test] DataArray: add test for renewed 'data' property
DataArray.create_data: Use data property to return the DataSet

Instead of create a new DataSet object
DataSet: Add 'shape' property to return data dimensions
DataSet: Add __array__ array 'interface' method
[test] Test for DataSet's __array__
[doc] Add DataSet to documentation
Merge pull request #29 from gicmo/data_io1

Change DataArray's data property to return a DataSet

Thanks