This function reads a dataframe with columns:
shortArmSize and longArmSize and optionally OTU and
returns a list with the A2 values for the OTUs
If several species present, use column "OTU".
$$A_{2} = \frac{sCL}{xCL}$$
(s = std dev, CL = chr. length, x = mean)
related to:
$$CV_{CL} = A_{2} * 100$$
(CV = coeff. var.)