title: Dask: nunique method on Dataframe groupBy
body: <p>I would like to know if it is possible to have the number of unique items from a given column after a groupBy aggregation with Dask. I don't see anything like this in the documentation. It is available on pandas dataframe and really useful. I've seen some issue related to this, but I am not sure it is implemented.</p>
  
  <p>Can someone give me some hints about this?</p>