title: pandas: add week dates with dataframe
body: <p>I have a df like, which has such rows:</p>
  <pre><code>    p_id    m_id    x_id    g_id    u_id
  0   2       NaN     1408    7       121
  1   3       1259    117     23      315
  2   3       1259    221     9       718
  3   3       1259    397     76      367
  </code></pre>
  <p>and two datetime objects:</p>
  <p>start_date:</p>
  <pre><code>datetime.datetime(2021, 5, 25, 0, 0)
  </code></pre>
  <p>end_date:</p>
  <pre><code>datetime.datetime(2021, 5, 29, 0, 0)
  </code></pre>
  <p>how do I get a df like, basically (adding the week-dates from start_date to end_date with each row):</p>
  <pre><code>    p_id    m_id    x_id    g_id    u_id  s_date
  0   2       NaN     1408    7       121   2021-05-25
  1   2       NaN     1408    7       121   2021-05-26
  2   2       NaN     1408    7       121   2021-05-27
  3   2       NaN     1408    7       121   2021-05-28
  4   2       NaN     1408    7       121   2021-05-29
  
  5   3       1259    117     23      315   2021-05-25
  6   3       1259    117     23      315   2021-05-26
  7   3       1259    117     23      315   2021-05-27
  8   3       1259    117     23      315   2021-05-28
  9   3       1259    117     23      315   2021-05-29
  .
  .
  15  3       1259    397     76      367   2021-05-25
  16  3       1259    397     76      367   2021-05-26
  17  3       1259    397     76      367   2021-05-27
  18  3       1259    397     76      367   2021-05-28
  19  3       1259    397     76      367   2021-05-29
  </code></pre>