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

def _combine_regional_coverage(in_bams, samplenames, chrom, start, end, work_dir, data):
    """
    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
    """
    dfs = [_calc_regional_coverage(bam, chrom, start, end, sample, work_dir, data) for bam, sample
           in zip(in_bams, samplenames)]
    return rbind(dfs)