Use absolute import in dipy/data/__init__.py

[quantshah]

Please refer to the original issue #969. Use absolute import in place of relative for all .py files inside /dipy. Take care not to change .pyx files

@arokem @Garyfallidis Does #969 apply to init files too? Please verify if this is needed.

/dipy/dipy/data/__init__.py:
   54                                 read_mni_template)
   55  
   56: from ..utils.arrfuncs import as_native_array
   57  from dipy.tracking.streamline import relist_streamlines
   58  

[quantshah]

Do not change init files