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 is possible for StateVariableDerivative to be an object like StateVariable

I see. `setStateVariableDerivativeValue()` seems appropriate then. Thanks for clarifying.