readme update obsolete option

[acquaregia]

Hi the line in
./sample_from_model.py -m gdb13_exp/models/model.trained.100 --with-likelihood should be changed into ./sample_from_model.py -m gdb13_exp/models/model.trained.100 --with-nll

or change it in the code to reflect the readme

and add the meaning of the acronym somewhere in the file negative log-likelihood (NLL)

Keep on with the great work! marco