Add Aggregate functions to DataSources (interface)

[doekenorg]

Making the DataSources more robust for future features; possibly even outside the realm of just a DataView, it would be nice to have aggregate functions based on the applied filters.

We already have a count() method by default, because we use it to calculate pagination.

interface AggregateDataSource extends DataSource {
	/** @return int|float */
	public function avg();

	/** @return int|float */
	public function min();

	/** @return int|float */
	public function max();

	/** @return int|float */
	public function sum();
}

This data could also be helpful to connect a data source to other components like a Chart, or a Summary.