title: Groupby specific hour interval per day
body: <p>I have a data frame that looks something like:</p>
  <pre><code>df = 
  
                       val1     val2
  date
  ----------------------------------
  2021-06-01 00:00     1        3
  2021-06-01 01:00     2        4
  2021-06-01 02:00     3        5
  2021-06-01 03:00     4        6
  2021-06-01 04:00     5        7
  2021-06-02 00:00     6        8
  2021-06-02 01:00     7        9
  2021-06-02 02:00     8        10
  2021-06-02 03:00     9        11
  2021-06-02 04:00     10       12
  2021-06-03 00:00     11       13
  2021-06-03 01:00     12       14
  2021-06-03 02:00     13       15
  2021-06-03 03:00     14       16
  2021-06-03 04:00     15       17
  </code></pre>
  <p>If I then do something like:</p>
  <pre><code>df_new = df.groupby(df.index.date).mean()
  </code></pre>
  <p>The resulting data frame will be:</p>
  <pre><code>df_new = 
  
                       val1     val2
  date
  ----------------------------------
  2021-06-01           3        5
  2021-06-02           8        10
  2021-06-03           13       15
  </code></pre>
  <p>However, I would like this to happen only for a specific time interval for each day. So in this case maybe only from <code>00:00-03:00</code> and not the <code>04:00</code> hour. Hence, the resulting data frame should instead be:</p>
  <pre><code>df_true = 
  
                       val1     val2
  date
  ----------------------------------
  2021-06-01           2.5      4.5
  2021-06-02           7.5      9.5
  2021-06-03           12.5     14.5
  </code></pre>