Fix initial assignement in form of storages

[Bachibouzouk]

Currently one cannot save the changes in a battery form because of hidden required fields ({'thermal_loss_rate': ['This field is required.'], 'fixed_thermal_losses_relative': ['This field is required.'], 'fixed_thermal_losses_absolute': ['This field is required.']}).

Those field were initialized under the hood but apparently this way of initializing no longer works and thus needed updating