Add support for multiple inverters and multiple batteries

[scruysberghs]

Use case inverter:
Multiple inverters but not all can be curtailed because the option is not there in the inverter or there are financial reasons not to curtail for that specific inverter even with negative pricing (GSC's still exist in Belgium)
Can mostly be worked around with a big virtual inverter and some fancy automations in Home Assistant but in case of one curtailing and not the other one emhass won't be able to to take into account what is actually happening.

Use case batteries:
Also here this can be worked around by adding multiple batteries and have emhass see it as one big battery and clever HA automations. But it gets complicated when batteries have differtent priorities like one connected to a hybrid inverter and another one being a plug in battery. Also total availble (dis)charging power would be depending on the SoC of the individual batteries when one is either empty or at 100% SoC. Having the option (like with deferable loads) to add more than one battery, each with their own set of parameters, would solve this.