title: Expand rows by date range having start and end in Pandas
body: <p>I'm working with a data set containing information on a phenomenon occurring during some time frames. I am given the start and end time of the event and its severity, as well as some other information. I would like to expand these frames over some larger time period by expanding the rows within set time periods and leaving the rest of the information as NaNs.</p>
  
  <p>Data set example:</p>
  
  <pre><code>                         date_end         severity   category
       date_start           
  2018-01-04 07:00:00  2018-01-04 10:00:00     12          1
  2018-01-04 12:00:00  2018-01-04 13:00:00     44          2
  </code></pre>
  
  <p>What I want is:</p>
  
  <pre><code>                     severity   category
       date_start           
  2018-01-04 07:00:00     12         1
  2018-01-04 08:00:00     12         1
  2018-01-04 09:00:00     12         1
  2018-01-04 10:00:00     12         1
  2018-01-04 11:00:00     nan       nan
  2018-01-04 12:00:00     44         2
  2018-01-04 13:00:00     44         2
  2018-01-04 14:00:00     nan       nan
  2018-01-04 15:00:00     nan       nan
  </code></pre>
  
  <p>What would be an efficient way of achieving such a result?</p>