title: filtering a pytables table on pandas import
body: <p>I have a dataset created with pytables that I am trying to import into a pandas dataframe.  I can't apply a <code>where</code> filter to the <code>read_hdf</code> step.  I'm on pandas '0.12.0'</p>
  
  <p>My sample pytables data:</p>
  
  <pre><code>import tables
  import pandas as pd
  import numpy as np
  
  class BranchFlow(tables.IsDescription):
      branch = tables.StringCol(itemsize=25, dflt=' ')
      flow = tables.Float32Col(dflt=0)
  
  filters = tables.Filters(complevel=8)
  h5 = tables.openFile('foo.h5', 'w')
  tbl = h5.createTable('/', 'BranchFlows', BranchFlow, 
              'Branch Flows', filters=filters, expectedrows=50e6) 
  
  for i in range(25):
      element = tbl.row
      element['branch'] = str(i)
      element['flow'] = np.random.randn()
      element.append()
  tbl.flush()
  h5.close()
  </code></pre>
  
  <p>Which I can import just fine into a dataframe:</p>
  
  <pre><code>store = pd.HDFStore('foo.h5')
  print store
  print pd.read_hdf('foo.h5', 'BranchFlows').head()
  </code></pre>
  
  <p>which shows:</p>
  
  <pre><code>In [10]: print store
  &lt;class 'pandas.io.pytables.HDFStore'&gt;
  File path: foo.h5
  /BranchFlows            frame_table [0.0.0] (typ-&gt;generic,nrows-&gt;25,ncols-&gt;2,indexers-&gt;[index],dc-&gt;[branch,flow])
  
  In [11]: print pd.read_hdf('foo.h5', 'BranchFlows').head()
    branch      flow
  0      0 -0.928300
  1      1 -0.256454
  2      2 -0.945901
  3      3  1.090994
  4      4  0.350750
  </code></pre>
  
  <p>But I can't get the filter to work on the flow column:</p>
  
  <pre><code>pd.read_hdf('foo.h5', 'BranchFlows', where=['flow&gt;0.5'])
  
  &lt;snip traceback&gt;
  
  TypeError: passing a filterable condition to a non-table indexer [field-&gt;flow,op-&gt;&gt;,value-&gt;[0.5]]
  </code></pre>