Routines for converting to and from "julian days". Uses
the Gregorian calendar.

datetojul:
   Calculates the julian day of a given date relative to a 
   reference date, and prints it.
   Input is year, month, day-of-month, and reference year,
   reference month and reference day-of-month.

   NB: starts at 0, so
      datetojul 2008 1 1 2008 1 1 
   returns  000000

jultodate:
   Calculates the date corresponding to a julian day relative
   to a given reference date.

   Input is julian day, reference year, reference month and reference
   day-of-month.

Knut Lisæter, 23. Nov 2008