group_by top N rows by group

Description

Another functionality useful in group by would be to return N rows by group.
Not sure how h2o syntax should look like here.
Example syntax in other solutions for "largest two v3 by id6":

```

#pandas
df.sort_values('v3', ascending=False).groupby('id6').head(2)

#R data.table
DT[order(-v3), head(v3, 2L), by=.(id6)]

#dplyr
DF %>% arrange(desc(v3)) %>% group_by(id6) %>% filter(row_number() <= 2L)

#python datatable
DT[:2, :, by(f.id6), sort(-f.v3)]
```

Assignee

New H2O Bugs

Fix versions

None

Reporter

Jan Gorecki

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Priority

Major
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