Write python code based on this description: "This is a class called "Source" in Python. The class has two methods:
1. getoutput: This method returns the value of the stored signal, which is initially set to None.
2. setvalue: This method takes an argument called value and stores it in the storedsignal attribute.
The class also has a method called connectsignalreader that takes an argument called signalreader and assigns it to the attribute stored_signal."