Default values for cost calculation dict

[FelixStege]

The default values for the cost_calculation from the indicator casue trouble in combination with json, since np.nan is not known in json:

const_electricity_price: float = Field(
        default=np.nan, description="constant electricity price in ct/kWh"
    )
    const_feed_in_price: float = Field(
        default=np.nan, description="constant feed-in price in ct/kWh"
    )

switch to None instead