Implement groupByFunction method

[roberto-butti]

Implement a groupByFunction method that accepts a closure as an input parameter, following the same logic that JavaScript’s Object.groupBy() uses. The idea is apply a callback (closure) to the Data, that defines how the elements should be grouped, and then return an associative array where the keys are the grouping results and the values are arrays of items that belong to each group.

[roberto-butti]

closed by #35