Add topic tree for pubsub messages

[alexdewar]

          It will be useful, I think, to indicate somewhere the list of all possible messages a particular device type handles - either sending or receiving. In the end, pubsub topics are implicit function calls with their own parameters and they should be documented in detail as we do with any other function.

We have never done it before, but given the complexity of the messaging system of FINESSE, it might be worth considering doing something like this https://pypubsub.readthedocs.io/en/v4.0.3/usage/usage_advanced_maintain.html#specify-topic-tree-def, although as most topics are created implicitely based on the device names, I'm not sure how far we can go down this route...

Originally posted by @dalonsoa in #379 (comment)