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.
>Would setStateVariableDerivative() be sufficient?

I was on the fence about it but opted for adding Value because it was also consistent with the method comment that says set "the derivative value" .