Lots of PyTables warnings

[louisabraham]

While converting the code to Python 3, I found strange warnings. In fact, the problem exists already at 0fe520e with Python 2 (before I started messing around with the encodings etc).

To reproduce:

• Execute test_analyze() modified to comment the line shutil.rmtree('test_items')
• In a Python interpreter:

>>> import pandas as pd
>>> store = pd.HDFStore('test_items/data.abx')
>>> store.info()

It will show a lot of warnings like:

/Users/louisabraham/miniconda3/envs/abxpy/lib/python2.7/site-packages/tables-3.4.2-py2.7-macosx-10.6-x86_64.egg/tables/group.py:1187: UserWarning: problems loading leaf ``/bys``::

  variable length strings are not supported yet

The leaf will become an ``UnImplemented`` node.
  % (self._g_join(childname), exc))
/Users/louisabraham/miniconda3/envs/abxpy/lib/python2.7/site-packages/tables-3.4.2-py2.7-macosx-10.6-x86_64.egg/tables/attributeset.py:299: DataTypeWarning: Unsupported type for attribute 'sorted' in node 'c2_v0'. Offending HDF5 class: 8
  value = self._g_getattr(self._v_node, name)
/Users/louisabraham/miniconda3/envs/abxpy/lib/python2.7/site-packages/tables-3.4.2-py2.7-macosx-10.6-x86_64.egg/tables/attributeset.py:299: DataTypeWarning: Unsupported type for attribute 'empty' in node 'c2_v0'. Offending HDF5 class: 8
  value = self._g_getattr(self._v_node, name)
/Users/louisabraham/miniconda3/envs/abxpy/lib/python2.7/site-packages/tables-3.4.2-py2.7-macosx-10.6-x86_64.egg/tables/attributeset.py:299: DataTypeWarning: Unsupported type for attribute 'sorted' in node 'c2_v1'. Offending HDF5 class: 8
  value = self._g_getattr(self._v_node, name)
/Users/louisabraham/miniconda3/envs/abxpy/lib/python2.7/site-packages/tables-3.4.2-py2.7-macosx-10.6-x86_64.egg/tables/attributeset.py:299: DataTypeWarning: Unsupported type for attribute 'empty' in node 'c2_v1'. Offending HDF5 class: 8
  value = self._g_getattr(self._v_node, name)
/Users/louisabraham/miniconda3/envs/abxpy/lib/python2.7/site-packages/tables-3.4.2-py2.7-macosx-10.6-x86_64.egg/tables/attributeset.py:299: DataTypeWarning: Unsupported type for attribute 'sorted' in node 'c2_v2'. Offending HDF5 class: 8
  value = self._g_getattr(self._v_node, name)
/Users/louisabraham/miniconda3/envs/abxpy/lib/python2.7/site-packages/tables-3.4.2-py2.7-macosx-10.6-x86_64.egg/tables/attributeset.py:299: DataTypeWarning: Unsupported type for attribute 'empty' in node 'c2_v2'. Offending HDF5 class: 8
  value = self._g_getattr(self._v_node, name)
/Users/louisabraham/miniconda3/envs/abxpy/lib/python2.7/site-packages/tables-3.4.2-py2.7-macosx-10.6-x86_64.egg/tables/group.py:1187: UserWarning: problems loading leaf ``/regressors/c2_v0/indexed_datasets``::

  variable length strings are not supported yet

[Thomas-Schatz]

I had some issues with newer versions of pytables, that I solved with:
conda install pytables=3.3.0

I think the long-term solution would be to try to remove the Pytable dependency altogether (it's not used a lot in the code) and rely only on h5py which is more performant and more stable.