Allow user to change parameters in MMGBSA.

[diegoenry]

Enable GBSA models igb=1,2,5,8 Implement PBSA (igb=RTFM)

Most GB/PB model parameters are auto-assigned during runtime (AMBER) for Minimization and MD simulations after "igb" is defined.

For MM(PB,GB)SA post-processing it is mandatory to modifty the TOPOLOGY using "ante-MMGBSA.py" . Make sure to use the proper setup for each model.