More flexible file loading in monitor/multi_datasets.py

[schlunma]

Description

Some files written by the preprocessor can have multiple variables (e.g., ones that contain a AuxFactory; this is desired by iris and not a bug). Currently, these files cannot be loaded in monitor/multi_datasets.py since it uses iris.load_cube(). This PR aligns this diagnostic with monitor.py and allows multi-variable input.

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[valeriupredoi]

very nice indeed, cheers, Manu 🍺