represent ensemble of structures as a density map.

See 
   C.D. Schwieters and G.M. Clore, ``G.M. Reweighted atomic densities 
   to represent ensembles of NMR structures,'' J. Biomol. NMR 23, 221-225
   (2002).


constructor:
  AtomProb(atomSel,
	   atomPosList) - 
    atomSel is a string or <m atomSel>.AtomSel
    atomPosList is a list of atomic coordinates for the structures which
      will make up the map.


methods:
  calc()             - calculate density map
  writeEDM(filename) - write map to specified file in xplor edm format

  accessor pairs:
  
  atomRadius/setAtomRadius
  verbose/setVerbose        - boolean
  fullMap/setFullMap        - boolean. Specify whether to calculate map due
			      to all atoms, rather than only those in the
			      selection.
  distType/setDistType      - one of "quartic","gaussian"
  scaleType/setScaleType    - one of "amplitude","distribution","flat"


members:
  gridVals - with members: 
  	xdelta, ydelta, zdelta
  	xmin, ymin, zmin
  	xmax, ymax, zmax        - grid parameters
  	cushion                 - space around molecule for auto-grid 
				  generation