title: Create dataframe from itertools product
body: <p>I have two lists:</p>
  
  <pre><code>a = [1,2,3]
  b = [4,5,6]
  </code></pre>
  
  <p>I want to create a dataframe whereby each combination of <strong>(a,b)</strong> generates a dataframe <strong>X</strong> and I pick out the max value of <strong>X</strong>, with the resulting output rows/columns with the elements in <strong>a</strong> and <strong>b</strong>.</p>
  
  <pre><code>df=[]
  
  for i, j in itertools.product(a, b):
      X = do_something(i,j)  ## this is a dataframe
      x_value = X.max()
      df.append(i,j,x_value)
  
  df=pd.DataFrame(df, columns=['a', 'b', 'x_value'])
  </code></pre>
  
  <p>The output dataframe should have columns as <strong>a</strong>, rows as <strong>b</strong>, and values as <strong>x_value</strong>.</p>
  
  <pre><code>    1   2   3
  4           
  5           
  6           
  </code></pre>