The x_by_b implementation tests whether a continuous variable's
distribution differs across the levels of one or more grouping
variables. It accepts one or more grouping variables via x_by_b(),
running one Cochran test per grouping variable.
Arguments
cqtest_def_xby's baseline fn takes no arguments beyond .proc.
Nothing is currently passed through ... in COCHRAN_QTEST().
Grouped Cochran's test default class
By default, returns a class_cq_test object.