Feature Request: Set SoC (Daly) #144

I've just started using this in a standalone Docker container and so far it's working great!

I would like to suggest adding the ability to set state of charge of the BMS.

This would be useful as my Daly BMS ignores current below ~2.5A, and resultantly after a few weeks the reported SoC is up to 20-30% off the real value. When I get a low voltage cut-off, it would be good to be able to reset the SoC to 0%, which I would normally do through the app but obviously cannot if batmon is currently connected to the BMS.

Great work, thanks again!