surface area calculator

 SurfaceArea objects are usually created using the create_SurfaceArea 
function from the <m surfaceTools> module.

constructor: 
  SurfaceArea(radii, atomSel)
   radii is list of length numAtoms containing atomic radii. These are 
   effective radii- any solvent radius should be pre-added.
   atomSelection is selection used in the surface area calculation.
	      

  singleAtom(atom, altRadius=-1., saveData =0)
   calculate and return exposed solvent accessible surface area for the 
    specified atom. Only those neighboring atoms included in selection
    are included in this calculation.
    An alternative radius for this one atom can be optionally specified.
    also information from a previous calc can be passed in as saveData
     - this will speed things up if the atoms haven't moved too much.

  singleAtomDerivs(DerivList&);
   calculate derivatives wrt surface area info. This must be called after
    singleAtom, and before atoms have moved.

  saveData()
    return the SaveData info corresponding to the most recent singleAtom() 
    call.


  radius(Atom)        setRadius(Atom, num)
    get/set the effective radius of the specified atom.

  selection()         setSelection(atomSel);
    get/set atoms to include in surface area calc.

  simulation()
    get the simulation associated with selection()

  verbose()           setVerbose(val)  
    get/set the verbosity level (0-4) [default:0]

  useConvexHull()     setUseConvexHull(val)
    get/set the value of useConvexHull. This specifies whether the Convex Hull
    neighbor culling algorithm is used. See Fraczkiewicz and Braun, J. Comp. 
    Chem 3, 319 (1998) for more details. [default:0(off)]

  useSphereMetric()   setUseSphereMetric(boolVal)
  sphereGridSize()    setSphereGridSize(intVal)
    sphere metric approximation: divide sphere evenly (by surface are)
    into grids. For each neighbor find the closest grid and calculate a
    metric based on how much surface of cAtom is occluded. For each grid point
    retain only that atom which has the maximum metric.
    thus, the number of neighbors is reduced. [off by default]

  moveTol()           setMoveTol(floatVal)
    how much movement to allow before recalcing neighbor identity [default: 0.]
  
  setModified()
    notify object that atom positions have changed.

  updateTouchingList()
    update the array of lists of touching atoms.