Creates an object composed of keys generated from the results of running
each element of collection thru iteratee. The order of grouped values is
determined by the order they occur in collection. The corresponding value
of each key is an array of elements responsible for generating the key.
The iteratee is invoked with one argument: (value).
_.groupBy([6.1, 4.2, 6.3], Math.floor);
// => { '4': [4.2], '6': [6.1, 6.3] }
// The `_.property` iteratee shorthand.
_.groupBy(['one', 'two', 'three'], 'length');
// => { '3': ['one', 'two'], '5': ['three'] }