make code agnostic to/intelligent around model_level_number, full_levels and half_levels

This is the second sub-issue:

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.

Search the codebase for model_level_number, full_levels, and half_levels, then trace the callers that interpret model levels for UM and LFRic. Map the affected paths before changing them; done means a generic model_level is handled correctly for both model conventions, with existing behavior preserved.

Written by the indexing model from the issue text.