Rename Component get/set to refelect returned variable value

Rename Component methods to reflect that the get/set method is accessing the value of StateVariable, CacheVariable, DiscreteVariable and not the object itself. This is now consistent with getOutputValue() and getInputValue() methods. Fixes issue #163.
> it isn't the derivative of the state variable that's being calculated, it's the derivative of the underlying function

We're just solving a differential equation v = integ(vdot(state)). So we don't have an explicit function for v. If it were quadrature then I think functionValue/derivativeValue would make sense.