Initial Support for PA series

[pp81381]

First cut of support for PA series

So far, I've followed the pattern of the SA series and supported the PA series to about the same extent as the SA series is supported. All of the known command codes can be sent and received by the Client but the State class can only be used for power and mute so far. (Worth noting that it is necessary to call update() on the State object before power or mute will work for the SA or PA series.)

I'm working on further changes to add support for decoding more of the messages and adapt the State class for it. Will write some ideas in the Issue that I posted.

[elupus]

Looks good so far, do you want me to merge this now?

[pp81381]

I'd say yes. I have more work in the pipe but it doesn't affect this piece and I'm not ready to commit it yet so I'll raise a new PR for that.