new nonbondenergy function in python that mask forcefield complexity

[benboyer]

Many script in vrac have a copy of a function ener (mini_MC_DNA.py, heliring.py,mini_MC.py,sampling_MC_metrop_prot_heligeom.py). They should be unified in a single function. This function should be able to accept the different forcefields, to automatically call the corresponding parameter files (aminon.par, scorpion.par,...) and to accept as optional argument the cutoff. ideally the nonbondenergy function should choose by itself the correct forcefield based on the reduced model used. This information is available in the reduced file but not yet represented in ptools. this should modified too, perhaps only at the python level.