Option to create an OpenMM system rather than context #315

For the GCMC work I need to use an OpenMM system to set the atomic properties on the GPU. To do so, I create a CPU context at the specified lambda value, then extract the required properties from the nonbonded forces in the system used to set up the context. This seems a little overkill since the context isn't needed, i.e. I'm not actually running a simulation. As such, it would be nice if there was a way to create an OpenMM system only, or to get the morphed parameters for each force.