access to native XPLOR potential terms

The XplorPot class allows access to potential terms written using the XPLOR
interface. The setup of these terms is performed within the XPLOR interface, 
or using the helper functions in the <m protocol> module. Analysis of most 
XPLOR terms is handled by the analyze function in the <m simulationTools>
module.

Constructor:
  XplorPot(NAME) 
   NAME is a string (maximum 4 characters) which represents an XPLOR 
   potential term. 

   NAME can also be a list of XPLOR term names, separated by the "+" character. 
   This is useful for certain terms, like PVDW, and VDW which do not evaluate
   correctly if evaluated separately.

Methods:

  only the standard potential terms exist:

  potName()      - returns 'XplorPot'
  instanceName() - returns NAME given in the constructor

  scale()            - get/set a scale factor for this term
  setScale(scaleFac) -  NOTE: this is independent of scalings set within
                        the XPLOR interface.

  calcEnergy()                   - calc energy without and with gradient
  calcEnergyAndDerivs(DerivList) -   evaluation. Return the energy value.

  rms()        - return XPLOR-calculated RMSD for this term, if it is defined 
                 in python/xplorPotTools.py
  violations() - return XPLOR-calculated number of violations for this term, if
                 it is defined in python/xplorPotTools.py
  numRestraints() - return XPLOR-calculated number of restraints for this term,
                    if it is defined in python/xplorPotTools.py