Quantitykind for State of Charge

[lazlop]

@steveraysteveray For working with batteries/thermal energy storage it may be useful to have a state of charge quantitykind, should I create a pull request for this?

[steveraysteveray]

Were you imagining something like a percentage as the applicable unit? Yes, this sounds OK.

[steveraysteveray]

(Actually, we do already have BatteryCapacity, but that is defined as the product of current and time (like ampere-minute). So we would need to differentiate the two concepts to make it clear. The state of charge multiplied by the BatteryCapacity would presumably tell you how many ampere-minutes are still available.)