XPweekYear()
 Return Year applying to Number return by WeekNr
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   Function:   XPweekYear()

   Syntax:     XPweekYear([dVar]) --> nYear

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

   Returns:    See XPweekNr(). It returns the Year where the week returned
               from XPweekNr() applies to. Generally same year, sometimes
               different.

   Usage:      XPweekYear(XPstod('19911229')) --> 1991
               XPweekYear(XPstod('19911230')) --> 1992      // next year
               XPweekYear(XPstod('19920101')) --> 1992      // this year
               XPweekYear(XPstod('19930103')) --> 1992      // 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.