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.
In theory it is possible for StateVariableDerivative to be an object like StateVariable is and used to encapsulate the derivative value and check its validity etc... (not unlike Output). Is it not a tiny tiny bit clearer that this is strictly a value? I am OK reverting. I have no strong desire either way.