code: def _combine_regional_coverage(in_bams, samplenames, chrom, start, end, work_dir, data):
      
      dfs = [_calc_regional_coverage(bam, chrom, start, end, sample, work_dir, data) for bam, sample
             in zip(in_bams, samplenames)]
      return rbind(dfs)
docs: given a list of bam files, sample names and a region, calculate the
      coverage in the region for each of the samples and return a tidy pandas
      dataframe of the format:
  
      chrom position coverage name