Helper functions to deal with week of the year and weekday according to ISO 8601.
This is an easy-to-use alternative to strftime(x, "%G-W%V") and a substitute for
strptime(x, "%G-W%V-%u") on input. In addition, the package offers functions
to convert from standard calender format yyyy-mm-dd to and from ISO 8601 week
format yyyy-Www-d.