Standard errors for 'relrisk.ppp' and minor changes.

   density.ppp.R
   smooth.ppp.R
   spatstat-internal.Rd
      Tweaks to internal code

   relrisk.R
   relrisk.ppp.Rd
      Algorithm completely rewritten.
      New argument 'se' allows calculation of standard errors.

   tests/density.R
      Extensive suite of tests of 'relrisk'.

   listof.R
   contour.listof.Rd
	image.listof has new argument 'ribmar'

   NEWS
   DESCRIPTION
      updated.