DO NOT MERGE - Charger Controller

[ducky64]

... because when you're at the race and need to repurpose the datalogger to send messages to tell the charger to go, here it is. Do not merge, this is only a PR to track changes.

I think there's a 1.2v offset on the voltage control - that is, the voltage control here needs to be set 1.2v below the actual setpoint as ready by BMS.

To change the setpoint, modify these lines:

        charger_control.voltage_be = swap16(115*10 + 7);  // 115.7V
        charger_control.current_be = swap16(10*10);  // 10A

The raw voltage is in 1/10 V units and the raw current is in 1/10 A units - hence the *10.