refactor HTF softcore_LJ to understand literals instead of bools

[ijpulidos]

We have been using Literal['gapsys', 'beutler'] in our AlchemicalSettings object. We probably want to handle this types directly in the HybridTopologyFactory class as well, instead of having to do things like

        if alchem_settings.softcore_LJ.lower() == 'gapsys':
            softcore_LJ_v2 = True
        elif alchem_settings.softcore_LJ.lower() == 'beutler':
            softcore_LJ_v2 = False

In the end we probably want to have an HTF object that is extensible for using different softcore potential functions in a friendly intuitive way.