Extract out aggregation functions

The group-by aggregator contains many different aggregation functions. These are currently copied into the Pivot plugin. This is bad since they will start to go out of sync.

At the very least, the aggregation functions should be refactored out into a separate module that both plugins use. It may also make sense to go one step further and make them plugins, though there are more considerations there, such as how to display the functions in the UI.