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

Empieza rastreando cómo se lee ExistingCapacity.csv y cómo se registran los activos cuando la capacidad del año inicial es cero. Reproduce el caso con solo capacidad futura y, después, verifica que el activo se registre y que se aplique la adición futura sin añadir una capacidad provisional en el año inicial.

Escrito por el modelo de indexación a partir del texto del issue.