add countDistinct on GroupBy

[devcrocod]

I would like to have a function countDistinct() on grouped dataframe for counting distinct values by group

df.groupBy { column_0 }.countDistinct()

df.groupBy { column_0 }.countDistinct { column_1 }

[Jolanrensen]

but then it should probably be named countDistinctValues() right?

[devcrocod]

No, it's similar https://kotlin.github.io/dataframe/countdistinct.html but on grouped df. I wouldn't like the naming to become more complicated

[devcrocod]

this is similar to other functions that apply to both a dataframe and a grouped dataframe: count, sum, mean ...