Formats a fixed-point field.

def format_float_field(__, prec, number, locale):
    """Formats a fixed-point field."""
    format_ = u'0.'
    if prec is None:
        format_ += u'#' * NUMBER_DECIMAL_DIGITS
    else:
        format_ += u'0' * int(prec)
    pattern = parse_pattern(format_)
    return pattern.apply(number, locale)