title: Making dataframe from a nested list
body: <p>Looking to convert :</p>
  
  <pre><code>nested_list = 
  [('R1',
    {'a', 'b', 'c'},
    {20.0,   40.0,   50.0,   60.0,   750.0}),
   ('R2',
    {'x', 'y', 'z'},
    {35.0,   37.5,   165.0}), 
   ('R3',
    {'x', 'a', 'm'},
    {2.5,   5.0,   7.5,   10.0,   12.5,   45.0})]
  </code></pre>
  
  <p>...into a dataframe as follows :</p>
  
  <pre><code>Cat   Column    Value
  ---   ------    -----
  R1    a         20.0
  R1    a         40.0
  R1    a         50.0
  R1    a         60.0
  R1    b         20.0
  R1    b         40.0
  ...
  R3    m         12.5
  R3    m         45.0
  </code></pre>
  
  <p>Each row of the list (e.g. R1) has a set of readings (like <code>{20.0, 40.0...}</code>) and set of elements (<code>{'a', 'b', ...}</code>). The readings and elements are of unequal size.</p>