Method aggregate

等同于 :func:`bigflow.transforms.aggregate(self, aggregate_fn, combine_fn, *side_inputs, **options) ` Args: pcollection (PCollection): 输入PCollection zero (value or function): 初始值，或是一个返回初始值的方法 accumulate_fn (

(self, zero, aggregate_fn, combine_fn, *side_inputs, **options)