Add start/stop charging register to modbus implementation

[sandervandegeijn]

I would like to start/stop charging through modbus. As I see it now, I can set the charging current, but cannot start/stop by a call over modbus.

[bobosch]

You could use the access bit 0xA7

[sandervandegeijn]

This is working, maybe somewhat cleaner than using the modes, but still not really a clean start/stop method. Dedicated functions for this would be nice.