title: elementwise operations on a multindex column failing due to join or level confusion
body: <p>I have a three-level multindex column   <code>Name, Period, Measurement</code>  In my toy example, I have weight and height for two time periods 'before' and 'after' for three different people.   </p>
  
  <p>I want to generate a dataframe that shows the <code>weight</code> divided by <code>height</code> for each of the three names during the <code>before</code> period.</p>
  
  <p>I am making some kind of error in my use of <code>level=</code> parameter I suspect (or perhaps .  But no matter what i try, I get a <code>TypeError: Join on level between two MultiIndex objects is ambiguous</code></p>
  
  <p>I would like to understand how to control operation so that i could specify, "show me, by name, where period='before', the weight divided by height"  and also the more general, "show me, by name, and by period, the weight divided by height"</p>
  
  <pre><code>arrays = [['mike', 'mike', 'mike', 'mike','matt', 'matt', 'matt', 'matt','dave','dave', 'dave','dave', ], 
            ['before', 'before', 'after', 'after']*3, 
           ['height', 'weight']*6]
  cols = pd.MultiIndex.from_arrays(arrays, names=('name', 'period', 'statistic'), )
  the_data = [[1.8, 200, 1.7,170]*3,[1.8, 190, 1.7,166]*3 ]
  idx = pd.IndexSlice
  xf = pd.DataFrame(index=pd.date_range(start="20191201", periods=2, freq="d"),
                           data=the_data, columns=cols)
  xf.loc[:, idx[:,'before','weight']].divide(xf.loc[:, idx[:,'before','height']], level=[0], axis='columns')
  </code></pre>