Add compute_wind_speed_if_missing to XarrayDataLoader

[copybara-service]

Add compute_wind_speed_if_missing to XarrayDataLoader

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[sylvesterkaczmarek]

variables is typed as Iterable[str], but with compute_wind_speed_if_missing=True it is consumed once by set(self._variables) and then again by list(self._variables). A valid one-shot iterable such as a generator is therefore empty on the second pass and can raise that none of the requested variables exist. Could the iterable be materialised once (for example in __init__) before both uses?