PyDataArray: add _create_data to create datasets
DataArrayMixin: Add create_data wrapper for _create_data

Handles various combinations of None arguments quite nicely
[test] DataArray: Test for .create_data()
[travis] Don't install (old) numpy via apt-get use pip instead
[test] DataArray.create_data: Test incompatible shape check

Where data.shape != shape
Merge pull request #23 from gicmo/create_data

Implement DataArray::create_data

Very nice