title: access the outer element of tuple in pandas dataframe groupby object
body: <p>I have a groupby object created on a multiindex dataframe (eg (a,b)). I can access the inner groups, with <code>df.get_group((a,b))</code>. However, i also want to be able to access the outer group, like <code>df.get_group((a,))</code>. I can achieve this indirectly through <code>df.xs(key,level)</code> command, but this is not the ideal way to solve the problem. Can somebody help me with this?</p>
  
  <pre><code>    df1.get_group(('specialty finance','mortgage finance'))
                                            lasts         prev ticker
  industry          sub_industry                                     
  specialty finance mortgage finance   190.000000   190.649994   dewh
                    mortgage finance   252.699997   253.350006   grhf
                    mortgage finance  1179.800049  1180.050049   hdfc
                    mortgage finance   693.849976   708.450012   ihfl
                    mortgage finance   450.950012   468.250000  lichf
  
  df1.get_group(('specialty finance',))
  ValueError: must supply a a same-length tuple to get_group with multiple grouping keys
  </code></pre>