Create common database for commands

[raacampbell]

We lack a centralised location for storing the message protocol. If the message format changes, we will need to manually modify each plugin. e.g. the constants class in MATLAB. Information is also scattered too much right now. e.g. the aforementioned class has no information on how to decode the returned status ("idle", "active", and "rampdown") from Zapit. That currently sits in TCPserver.

I propose having this information in a text file organised in some clear manner. MATLAB and Python can then each have a class that reads from this file and presents its content in a way similar to what is now done by the constants class. Presumably something analogous is possible for Bonsai. But even if it's not, having all the information in a single language-agnostic place is a bonus.

There should be instructions on how to add or modify commands to the protocol. These instructions should explain what changes need to made both here and in the zapit.interfaces.TCPserver package.