Instances of MovablePointChargeModel have both position and positionProperty

[jbphet]

While working to resolve a bug reported by QA (#547), I found that instances of MovablePointChargeModel have a property named position and another named positionProperty. The latter is added to the base class PointChargeModel via inheritance. This seems like poor design, and confused me while working on it, and led me to create the aforementioned bug. I don't have the time to fix this at the moment, but the next time any significant work is done on this sim, this should be addressed, either by improving the design or at least the naming.