docs: def estCumPos(pos,chrom,offset = 20000000):
      '''
      compute the cumulative position of each variant given the position and the chromosome
      Also return the starting cumulativeposition of each chromosome
  
      Args:
          pos:        scipy.array of basepair positions (on the chromosome)
          chrom:      scipy.array of chromosomes
          offset:     offset between chromosomes for cumulative position (default 20000000 bp)
      
          Returns:
          cum_pos:    scipy.array of cumulative positions
          chrom_pos:  scipy.array of starting cumulative positions for each chromosme
      '''
      chromvals = SP.unique(chrom)#SP.unique is always sorted
      chrom_pos=SP.zeros_like(chromvals)#get the starting position of each Chrom
      cum_pos = SP.zeros_like(pos)#get the cum_pos of each variant.
      maxpos_cum=0
      for i,mychrom in enumerate(chromvals):
          chrom_pos[i] = maxpos_cum
          i_chr=chrom==mychrom
          maxpos = pos[i_chr].max()+offset
          maxpos_cum+=maxpos
          cum_pos[i_chr]=chrom_pos[i]+pos[i_chr]
      return cum_pos,chrom_pos