we don't overload the operator anymore

[mnky9800n]

so code like:

    def __add__(self, other):
        if isinstance(other, TimeSeries):
            ts1 = TimeSeries(self.data, self.agg_func)
            ts2 = other
            tf = TimeFrame(ts1) + TimeFrame(ts2)
            return tf
        else:
            raise ValueError('You can only add TimeSeries objects to TimeSeries objects.')

should be removed.