Add a couple of spare bytes to the response message format
Only attempt to update trim value if power is non zero

Should enable training without a power meter, but limited to
the accuracy of the calibration data.
Add support for offline / manual mode

Pressing the plus or minus buttons when there is no workout active
will enter a manual mode where the resistance is adjusted up/down
by the buttons only. Pressing the cancel button or starting a new
workout will return to normal slope/ergo operation.

Note that in this manual mode, there is no automatic return to
minimum resistance (as there is no speed tracking).