Added a simple class 'yardstick' to display scale info.

   yardstick.R [New file]
   yardstick.Rd [New file]
   plot.yardstick.Rd [New file]
   spatstat-internal.Rd
      Simple class 'yardstick' to display scale info
      (moved from code in book)

   NEWS
   DESCRIPTION
	updated.