function to calculate Cpk - "measure of process centering"
Ppk(LSL, USL, QC.Center, s.Sigma)
number, customer's lower specification limit.
number, customer's upper specification limit.
number, the mean or median value determined from an XmR plot or a Studentized (e.g., xBar) analysis.
number, the sigma value determined from overall standard deviation (i.e., sd()).
numeric, Ppk value (unitless)