XPweekNr()
 Return Week Number according to ISO standards
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   Function:   XPweekNr()

   Syntax:     XPweekNr([dVar]) --> nWeek

   Arguments:  dVar is a date, defaults to date() (ie. today).

   Returns:    The week number according to standards set out by ISO.
               Note that at the end of a year, one can be in week 1, this
               then means that it is week 1 of the next year. Same applies
               to the beginning of the year, where one can be in week 52 or
               53, which indicates that week from the previous year.

               See the WeekYear() function, which indicates in which
               year the numbering applies to. Generally same year, but
               sometimes, it differs.

   Usage:      XPweekNr(XPstod('19911229')) --> 52      // last week
               XPweekNr(XPstod('19911230')) --> 1           // of next year
               XPweekNr(XPstod('19920101')) --> 1           // this year
               XPweekNr(XPstod('19930103')) --> 53          // previous year

   Note:       Credits go to Robert Franklin Lamping for pointing me
               out the ISO standards "Numbering of weeks" in an Article,
               and for the Clipper Summer '87 demo code illustrating the
               concepts. The standard in question is ISO 2015 1976.