Existing capacity not registered if zero capacity in the starting year

The modeler may want to specify capacity addition in a future year (e.g. if they know a power plant is going to be built in the future). This can be done through the `ExistingCapacity.csv` file, however anything put in this file will only be registered if there is nonzero capacity in the starting year. If this isn't the case, the asset won't be registered and any additional capacity put in future years won't be applied.

I think maybe a current workaround is to put a small capacity in the starting year (e.g. 0.1), (although I'm not sure whether that actually works), but it would be good to get this working without having to do that