8.4.11 basePolySubscan(self,channel=-1,phase=-1,order=1,subtract=1,plot=0):

basePolySubscan $<$ DataAna(BoaDataEntity.DataEntity): $<$ BoaDataAnalyser.py

    NAM: basePolySubscan (method)  
    DES: polynomial baseline method, treat each subscan. 
    INP: 
         (i) channel  =  -1: all channels; 0=first channel
         (i) phase    =  -1: all; else: 0,1,2
         (i) order    =  polynomial order, >0 
         (l) subtract =  1 or 0; subtract fit from data?
         (l) plot     =  1 or 0; overwrite Data_Red with fit?
    OUT: (f) data     =  overwrites data array with fit or data-fit
         (f) poly     =  polynomial coefficients are printed but not
                         returned, needs to be implemented