Improve cache variable usage in ExpressionBasedCoordinateForce

[nickbianco]

ExpressionBasedCoordinateForce requires that computeForce() is called before getForceMagnitude() since the cache variable must be updated first. It would be better to recalculate the cache variable within getForceMagnitude() as needed to decouple these methods.

Note that other Forces may have similar dependencies that should also be resolved.