title: use .lt(number).groupby().transform('all') without NaN
body: <p>Hel lo, I have a comman that allow mee to only keep groups were values in one column is lesse than 20.
  But sometime the values can be NaN and even if all the values are less than 20 this group is droped.</p>
  
  <p>I use this command: </p>
  
  <pre><code>tab=tab[tab['Event'].lt(20).groupby(tab['Clustername']).transform('all')]
  </code></pre>
  
  <p>How could I avoid to take into account the NaN values such as </p>
  
  <pre><code>Clustername                                              query  Event
  883137  Cluster84095     scaffold_4869:1428-4284(+):sp29    5.0
  883138  Cluster84095     scaffold_4869:1428-4284(+):sp29    5.0
  883139  Cluster84095     scaffold_4869:1428-4284(+):sp29    5.0
  883140  Cluster84095     scaffold_4869:1428-4284(+):sp29    5.0
  883141  Cluster84095   scaffold_1254:1696-5479(+):sp1    2.0
  883142  Cluster84095   scaffold_1254:1696-5479(+):sp1    2.0
  883143  Cluster84095   scaffold_1254:1696-5479(+):sp1    2.0
  883144  Cluster84095   scaffold_1254:1696-5479(+):sp1    2.0
  883145  Cluster84095        scaffold_4174:4132-5446(+):sp2    NaN
  883146  Cluster84095        scaffold_4174:4132-5446(+):sp2    NaN
  883147  Cluster84095        scaffold_4174:1944-3504(+):sp2    NaN
  883148  Cluster84095      scaffold_2714:1163-4220(+):sp4    5.0
  883149  Cluster84095      scaffold_2714:1163-4220(+):sp4    5.0
  883150  Cluster84095      scaffold_2714:1163-4220(+):sp4    5.0
  883151  Cluster84095      scaffold_2714:1163-4220(+):sp4    5.0
  883152  Cluster84095  IDBA_scaffold_2184:3542-6539(+):sp7   4.0
  883153  Cluster84095  IDBA_scaffold_2184:3542-6539(+):sp7   4.0
  883154  Cluster84095  IDBA_scaffold_2184:3542-6539(+):sp7   4.0
  883155  Cluster84095  IDBA_scaffold_2184:3542-6539(+):sp7   4.0
  883156  Cluster84095  IDBA_scaffold_1712:13782-16775(-):sp9    4.0
  883157  Cluster84095  IDBA_scaffold_1712:13782-16775(-):sp9    4.0
  883158  Cluster84095  IDBA_scaffold_1712:13782-16775(-):sp9    4.0
  883159  Cluster84095  IDBA_scaffold_1712:13782-16775(-):sp9    4.0
  883160  Cluster84095  scf7180005173345:6698-9667(-):sp12    4.0
  883161  Cluster84095  scf7180005173345:6698-9667(-):sp12    4.0
  883162  Cluster84095  scf7180005173345:6698-9667(-):sp12    4.0
  883163  Cluster84095  scf7180005173345:6698-9667(-):sp12    4.0
  883164  Cluster84095     JZSA01000885.1:34358-37408(-):sp3    3.0
  883165  Cluster84095     JZSA01000885.1:34358-37408(-):sp3    3.0
  883166  Cluster84095     JZSA01000885.1:34358-37408(-):sp3    3.0
  883167  Cluster84095     JZSA01000885.1:34358-37408(-):sp3    3.0
  917159  Cluster84095                                                NaN    1.0
  </code></pre>