check if files are fastq(.gz) and paired

def _check_paired(files, force_single, separators):
    """check if files are fastq(.gz) and paired"""
    full_name = _check_stems(files)
    if files[0].endswith(".bam"):
        return files
    elif is_gsm(files[0]):
        return files
    return combine_pairs(files, force_single, full_name, separators)