New api for DataFrame.groupBy() provided by GroupedDataFrame

[Gmousse]

A new api for groupBy will coming soon.

.groupBy() now returns a GroupedDataFrame object.

GroupedDataFrame provides a groupBy on multiple columns ( #4 ) and a method .aggregate() used to apply a function on each group.

example: df.groupBy('column1', 'column2').aggregate(group => group.count())

The aggregation returns a DataFrame with columns: ['column1', 'column2', 'aggregation']

[Gmousse]

The new api will be more accurate while applying groupBy on NaN and undefined (Object.is instead of ===)