title: Time range to index in pandas
body: <p>For each datapoint (in this example a and b), its specific occurrence time frame is specified:</p>
  <pre><code>pd.DataFrame([
      {'from': pd.Timestamp('2020-01-01'), 'to': pd.Timestamp('2020-01-05')}, 
      {'from': pd.Timestamp('2020-01-02'), 'to': pd.Timestamp('2020-01-03')}
  ], index=['a', 'b'])
  </code></pre>
  <p>I would like to count, for example, when the data point is active over time:</p>
  <pre><code>2020-01-01    1
  2020-01-02    2
  2020-01-03    2
  2020-01-04    1
  2020-01-05    1
  </code></pre>
  <p>Is there any convenient way to merge a period (timeframe) to a <code>DateTimeIndex</code>, for convenient plotting or counting?</p>