Adds file handler to logger.

    File is opened in "a" mode (append)

def add_file_handler(logger, logFilename=None):
    """Adds file handler to logger.

    File is opened in "a" mode (append)
    """
    assert isinstance(logger, logging.Logger)
    ch = logging.FileHandler(logFilename, "a")
    # ch.setFormatter(logging._defaultFormatter) # todo may change to have same formatter as last handler of logger
    ch.setFormatter(_fmtr)
    logger.addHandler(ch)