Parse the number of expected reads from the RunInfo.xml file.

def _expected_reads(run_info_file):
    """Parse the number of expected reads from the RunInfo.xml file.
    """
    reads = []
    if os.path.exists(run_info_file):
        tree = ElementTree()
        tree.parse(run_info_file)
        read_elem = tree.find("Run/Reads")
        reads = read_elem.findall("Read")
    return len(reads)