Design for discovery agents done

Updated plan for next steps

Can now construct a device class instance from its product_id

Default incoming message callback added to router, for logging/debug

Router unknown message handling callback registration is cleaner

Neater callback mechanism for device class instances to use. ...

This new architecture ensures default message processing always happens

Fixed bug where sensorid was always zero in csv log

Synthetic receive working in monitor_mihome.py

Updated TODO list with next steps for receive pipeline

Router ready to be tied into a loop()

control_both.py working in simulation

switch.py renamed as control_mihome.py, rewritten for new OO interface

legacy.py rewritten to use new device class interface

Updated TODO list for the next phase of work on device_classes branch ...

Specifically re-testing samples, then pushing OO classes up into app.