Added tests for zUnion and zInter with aggregate functions but without weights
Allow weights array to be null in generic_z_command (zUnion/zInter)

This way we can pass an aggregate function without having to also pass a dummy array(1,1,1..) with weights.
Merge remote-tracking branch 'mheijkoop/zintersection_optional' into feature/zintersection_optional
Merge branch 'feature/zintersection_optional' into develop