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.
IMHO, "DerivativeValue" is redundant—unless there could conceivably be a method called `setStateVariableDerivative()` that does something different. I couldn't imagine such a method. Regardless, feel free to merge if you like it.