Implement new belief abstraction: beliefs depending on other beliefs

[TimKam]

Currently, beliefs in JS-son are JSON objects and there are no dependencies between beliefs, at least not as first-class abstractions. Extend JS-son beliefs so that a belief can be a function of other beliefs. Here, we need the following (initial idea):

1. A belief can have a function, where the input variables are other beliefs, either beliefs in the agent's belief base or beliefs in the update that the agent receives.
2. The belief's value is used as a backup in case any of the input variables is undefined.
3. Beliefs with functions are ordered, i.e. they have an order number assigned that determines the order with which belief functions are called. Beliefs that are simple values are updated before the functions are called.