Addition of forcefield version selection for GAFF and SMIRNOFF #162
Description
Describe the feature you would like to have
Currently it is the case that for both gaff and openff the small molecule force fields are hard coded in generate_forcefield and generate_transitional_forcefield it would be good to make a separate function for them, with adding the versions and adding them in OpenMMDL Setup for the user to select, with the default being the latest version.