title: subset (slice) of pandas.DataFrame has the same index as the original DataFrame
body: <p>Suppose, I have a DataFrame with MultiIndex like this:</p>
  
  <pre><code>In [1]:d=pnd.DataFrame(range(5),index=pnd.MultiIndex.from_tuples([('A',1),('A',2),('A',3),('A',4),('A',5)]))
  
  In [2]: d
  Out[2]: 
       0
  A 1  0
    2  1
    3  2
    4  3
    5  4
  </code></pre>
  
  <p>I can create another DataFrame by subsetting:  </p>
  
  <pre><code>In [3]: p=d.loc[('A',slice(1,3)),:].copy()
  
  In [4]: p
  Out[4]: 
       0
  A 1  0
    2  1
    3  2
  </code></pre>
  
  <p>but the index object of this new DataFrame is the same as from the original DataFrame (contains all original items in the 'levels').</p>
  
  <pre><code>In [5]: p.index
  Out[5]: 
  MultiIndex(levels=[[u'A'], [1, 2, 3, 4, 5]],
             labels=[[0, 0, 0], [0, 1, 2]])
  </code></pre>
  
  <p>How do I copy-out a subset which does not 'remember' the index object of the original DataFrame? </p>
  
  <p>The reason I need to this is because some of my functions access the index object to get metadata, and the fact that the index carries over form the original DataFrame confuses these functions. </p>