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.
If you want to get technical, it isn't the derivative of the state variable that's being calculated, it's the derivative of the underlying function. From this perspective, the methods would be called `setStateVariableFunctionValue()` and `setStateVariableDerivativeValue()`.