toString of grouped data-frames should include variables, number of groups, and number of records

In R's toString implementation of grouped data-frames, the grouping attributes are displayed along with the complete table structure providing a much more informative view:
```
> iris %>% group_by(Species)
# A tibble: 150 x 5
# Groups: Species [3]
Sepal.Length Sepal.Width Petal.Length Petal.Width Species

1 5.1 3.5 1.4 0.2 setosa
2 4.9 3 1.4 0.2 setosa
```

In comparison, with kdf we can hardly imagine what's behind a grouped data-frame (making subsequent aggregations and transformation harder to work out interactively)
```
> irisData.groupBy("Species").print()
Species group
0 setosa [50 x 5]
1 versicolor [50 x 5]
2 virginica [50 x 5]
```
Expected: A grouped-df should render the same as an ungrouped table, but with additional information about the grouping attributes and the number of groups.

See https://dplyr.tidyverse.org/reference/group_by.html#ref-examples