title: Create series based on two pandas dataframe bool columns
body: <p>How do I create a series based on two pandas dataframe bool columns?</p>
  <pre><code>    round_up  round_down is_round_up is_round_down High     Low
  0   0.75       0.7       False       True          0.70532  0.69818
  1   0.75       0.7       False       True          0.70196  0.67268
  2   0.75       0.7       False       True          0.71243  0.69938
  3   0.75       0.7       False       True          0.70226  0.69884
  4   0.75       0.7       False       True          0.70292  0.69952
  5   0.75       0.7       True        True          0.75100  0.69000
  </code></pre>
  <p><strong>Desired output</strong> is a series with <code>round_up</code> if <code>is_round_up</code> or <code>round_down</code> if <code>is_round_down</code> or if both <code>True</code> select <code>is_round_up</code>.</p>
  <pre><code>0 0.70
  1 0.70
  2 0.70
  3 0.70
  4 0.70
  5 0.75
  </code></pre>
  <p><strong>Test data</strong></p>
  <pre><code>df = pd.DataFrame({'round_up': {0: 0.75,
    1: 0.75,
    2: 0.75,
    3: 0.75,
    4: 0.75,
    5: 0.75},
   'round_down': {0: 0.70,
    1: 0.70,
    2: 0.70,
    3: 0.70,
    4: 0.70,
    5: 0.70},
   'is_round_up': {0: False, 1: False, 2: False, 3: False, 4: False, 5:True},
   'is_round_down': {0: True, 1: True, 2: True, 3: True, 4: True, 5:True},
   'High': {0: 0.70532, 1: 0.70196, 2: 0.71243, 3: 0.70226, 4: 0.70292, 5:0.751},
   'Low': {0: 0.69818, 1: 0.67268, 2: 0.69938, 3: 0.69884, 4: 0.69952, 5:0.69}})
  </code></pre>
  <p>Edit: To clarify if both columns are <code>False</code> I would expect to see <code>Nan</code> for that row.</p>