Check if the current representation is a negavtive value.
This function allows to figure out if the given value is a negative value.
As it happens with afdal_decimal_is_positive, this function doesn't support any operational. See this function to know more about this issue.
Of course, the number before being checked for positiveness, is normalized so every representation implemented can use this function.
- TRUE if the number is positive or FALSE if not or it fails.