Fix FutureWarnings in pandas

At some point, we should fix the warnings I get when running pytest:

FutureWarning: Setting an item of incompatible dtype is deprecated and will raise an error in a future version of pandas. Value '49.9' has dtype incompatible with int64, please explicitly cast to a compatible dtype first.

Seems related to setting changing the value in a dataframe in lines such as

forecast_prices.loc[horizon[0], "e_price"] = price

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