Enable comparison of UM and LFRic data on model levels and not just pressure levels

Currently vertical model levels between UM and LFRic do not align height-wise.
In addition the UM uses model_level_number for vertical model levels, whereas LFRic uses full_levels and half_levels.
Therefore, we can currently only compare vertical information between UM and LFRic on pressure levels and not on model_levels. Due to this restriction any recipes that use model levels call for model_level_number and therefore do not work with LFRic and cause a can not generate a cube error as the constraint for model levels calls for model_level_number and not full_levels or halflevels.

Solution:
- 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.
- in a second step work is required in the code to enable a generic model_level variable that is interpreted as either model_level_number for UM or full_level or half_level for LFRic.
- In a third step work is required to understand how model_levels compare between Um and LFRic and ensure the code aligns them correctly for a direct comparison at the same height.

The work will be split up into 3 sub issues.