enable usage of different model level names such as model_level_number, full_levels and half_levels

UM and LFRic use different names for model levels. We therefore require more flexibility in our loader recipes.

modify loader recipes to allow the user to select model_level_number, full_levels or half_levels in the rose gui ONLY FOR MODEL LEVEL fields. This will still not allow a comparison between UM and LFRic on model levels, but will allow the user to run UM or LFRic only model_level based analysis.