The ProgressorGroup class calculates the arithmetic mean of all the progress values of all the referenced Progressor
Progressor Groups are useful if you need the mean progress value of two or more progress values.
E.g. Progressor Group (PG) has two Progressors (P1 and P2). If P1 progress = 1f and P2 progress = 0f then PG progress = 0.5f.