Consider replacing the use of label_molecules() by create_openmm_system() from OFF's ForceField

[martimunicoy]

Description

create_openmm_system runs the parameter assignment for OpenMM and it might be a safer option than label_molecules(). For example, the latter does not care about aspects such as exclusion lists (see issue #24).

Solution

Check the differences between both methods before performing any substitution. In case, it happens to be a better method to obtain the parameters for PELE, replace it in: toolkits.py