Allow switching relay(s) when CCL=0 or DCL=0 sent by managed battery

[izak]

Background: Some batteries are losing the physical allow-to-charge and allow-to-discharge relays. They use only CAN-bus communications for this.

In such systems, where there might be other equipment that still relies on an analog switch to control them.

Solution: Make it possible to switch the relays on the GX device so that these can be used instead.

[mpvader]

There are two open questions about this:

1. As implemented now, this feature only works with a BMS which sends CCL & DCL values. Which is confusing, since the gui now shows the settings always - even if you only have a BMV.
2. This code depends on CCL being zero, but, we are all the time telling BMS makers that they should never set CCL to 0...

And for my self: why is this feature relevant? Some use cases:

1. DC-Loads: while AC-Loads are taken of since the inverter or multi will be switched off, DC-loads are not. So for an off-grid system having DC-Loads, you could switch a Smart BatteryProtect with a GX relay, quite nice. about this feature is that (with systems having such BMS-es)
2. For example switch other charge sources that are not CAN controlled, such as a wind generator or alternator and so forth.

Lastly, thinking about this, there are pure Victron systems that need our attention as well, perhaps better to focus on that first?

1. A system with a smallBMS (ATC and ATD outputs, no comms port) a GX Device and an MPPT: since the VE.Direct port of the MPPT is connected to the GX Device, there is no way to have that switched on and off by the BMS. Solution: wire the digital outputs of the smallBMS into the digital inputs of the Cerbo GX, and control the MPPT by software.
2. A system with a VE.Bus BMS (ATC and ATD outputs, and VE.Bus), a GX Device and a MPPT. same problem, but here you could consider using the VE.Bus BMS connection via the inverter/charger, rather than the digital input wiring. But if thats a good idea, I don't know: what happens if the inverter/charger is off. .. deadlock situation etc.