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

まず、ExistingCapacity.csv がどのように読み込まれ、開始年の容量がゼロの場合にアセットがどのように登録されるかを追跡します。将来の容量のみがあるケースを再現し、その後、開始年に回避策として容量を追加することなく、アセットが登録され、将来の追加が適用されることを確認します。

索引モデルが issue の本文から書いたものです。