Added PBL data for detection of pbl with nuclei
Added empirical threshold for PBL with and without nucleus. First, the
median of the log10 distribution of the MeanIntensity DAPI is determined.
Then an empirical CV of 15% is assumed and 2*SDs are subtracted. This
gives a good threshold in 95% of the cases. Should of course be ignored
for non-adherent cells that all have a nuclei, e.g. HL60 or U936.