Bugfixes and new vignette 'datasets'.

   util.R
      Bug fix in internal function 'simplenumber'.

   psp.R
      Bug fix in plot.psp (ignored show.all in some cases)

   vignettes/hexagon.eps [New file]
   vignettes/hexagon.pdf [New file]
   vignettes/irregpoly.eps [New file]
   vignettes/irregpoly.pdf [New file]
   vignettes/datasets.Rnw [New file]
       New vignette summarising all datasets installed with spatstat.

   NEWS
   DESCRIPTION
       updated.