title: Breakdown data-frame into second-by-second time-series
body: <p>I have this dataset of active subjects during specified time-periods.</p>
  <pre><code>    start    end name
  0   00:00  00:10    a
  1   00:10  00:20    b
  2   00:00  00:20    c
  3   00:00  00:10    d
  4   00:10  00:15    e
  5   00:15  00:20    a
  </code></pre>
  <p>The intervals are inclusive on the left(start) side and not inclusive on the right(end).</p>
  <p>There are always three subjects active. I want to increase the granularity of the data, so that I will have info of the three active subjects for each second. Each second has three unique values.</p>
  <p>This would be the desired result for the test case.</p>
  <pre><code>   slot1 slot2 slot3
  0      a     c     d
  1      a     c     d
  2      a     c     d
  3      a     c     d
  4      a     c     d
  5      a     c     d
  6      a     c     d
  7      a     c     d
  8      a     c     d
  9      a     c     d
  10     b     c     e
  11     b     c     e
  12     b     c     e
  13     b     c     e
  14     b     c     e
  15     b     c     a
  16     b     c     a
  17     b     c     a
  18     b     c     a
  19     b     c     a
  </code></pre>
  <p>The order of the subjects inside the slots is irrelevant for now. The subjects can reappear in the data like &quot;a&quot; from 00:00 to 00:10 and then again from 00:15 to 00:20. The intervals can be at any second.</p>