Implement calc_check_digits() in IBAN
Implement calc_check_digits() in IBAN

Introduce a function to calculate the two check digits of an IBAN. Since
the check digits are the third and fourth characters in the number,
placeholders need to be provided when calling this function.

arthurdejong/python-stdnum,arthurdejong/python-stdnum,holvi/python-stdnum,arthurdejong/python-stdnum,holvi/python-stdnum,holvi/python-stdnum