title: multi-index and multi-column grouping
body: <p>I have a dataframe with 3 levels index and 2 level columns.</p>
  
  <pre><code>                                       Group
                             Label       A        B       C        D
  number      start          end              
  1           2020-01-01  2020-12-31  -43.0    0      105.0   -37.0
              2020-12-15  2020-12-15  NaN     NaN      NaN    195.0
  2           2019-01-01  2019-12-31  -35.0   80.0    -14.0   NaN
              2019-12-17  2019-12-17  NaN     NaN      NaN    141.0
              2020-01-01  2020-12-31  -15.0   45.0    -7.0    NaN
  3           2020-12-17  2020-12-17  NaN     NaN      NaN    326.0
              2022-01-01  2022-12-31  NaN     50.0     NaN    NaN
              2023-12-31  2023-12-31  -25.0   NaN      NaN    NaN
              2023-01-01  2023-12-31  NaN    50.0      NaN    NaN            
              2020-12-15  2020-12-15  NaN     NaN      NaN    61.0
  .............
  </code></pre>
  
  <p>I would like to group by number and start (only the year), summing values per Label:</p>
  
  <pre><code>                                      Group
                             Label       A        B       C        D
  number      start          end              
  1           2020        2020        -43.0    0      105.0   232.0
  2           2019        2019        -35.0   80.0    -14.0   141
              2020        2020        -15.0   45.0    -7.0    NaN
  3           2020        2020        NaN     NaN      NaN    387.0
              2022        2022        NaN     50.0     NaN    NaN
              2023        2023        -25.0   50.0     NaN    NaN    
  .............
  </code></pre>
  
  <p>Please note that there is higher-level-column as well (called Group, and other higher-level-columns that I am not including to keep it simple) and other sub-columns (Label: A, B, C, D, repeated for each higher-level-column). 
  how can I do this?
  thank you in advance</p>