Return the step value in format suitable for display.

def format_step(step, zero_prefix=False):
    """Return the step value in format suitable for display."""
    if isinstance(step, int):
        return "{:06}".format(step) if zero_prefix else "{}".format(step)
    elif isinstance(step, tuple):
        return "{:04}:{:06}".format(*step) if zero_prefix else "{}:{}".format(*step)