ensemble average a potential term

Used with an <m ensembleSimulation>.EnsembleSimulation for an energy
term which is a simple ensemble average of an energy term. i.e. the
potential is 

      E = sum_i w_i E_i

where the sum is over all members of an ensemble, w_i is the weight
for ensemble member i, and E_i is the averaged energy.

Constructors:

   AvePot( otherPotentialName, args )

otherPotentialName could be e.g. <m xplorPot>.XplorPot, <m
noePot>.NOEPot, etc. args are the arguments which these other terms
take. 

*or*

   AvePot( pot )

where pot is a previously defined potential term.

Methods:

  calcEnergy()
  calcEnergyAndDerivs()

  potName()
  instanceName()

  subPot() - return <m pot>.Pot object which is averaged over.

Accessors (read/write)
  scale - scale factor for the term. Overall scale is product of this value
          and subPot->scale().

Additionally, all of the methods of the underlying class can be accessed as if
the AvePot object was actually the sub-potential type.