[Feature] Allow aggregating over multiple time series

[lutter]

Description

Aggregations right now can only be done over a single timeseries. It would be good to allow aggreagations from multiple timeseries. This won't be a problem as long as each aggregate only uses data from one timeseries, but could be more difficult if you can mix multiple sources in the same aggregate, e.g. @aggregate(fn: "sum" arg: "Source1.priceUSD * Source2.amount"), as that will require aligning the two timeseries, i.e., answering the question 'which entry in Source2 corresponds to this entry in Source1')