2D Difference / bias map plots

[jfrost-mo]

Difference between two spatial fields.

• Plot 2 models side by side  #529
• Adding the model info to cube metadata at read time. #939
• Handle CubeLists in all operators that operate on Cubes #721
• Difference does something intelligent with metadata over just the subtraction operator.
• Add workflow bit to run difference plots and also line plots that take all models.
• UI work to set the base model for differences.
• Single diff button for all of the different plot types? (Spatial, line plots, etc.)

[jfrost-mo]

Consider around difference between cubes and differences between just the data in the cubes. This is relevant for comparing between UM and LFRic.

[jfrost-mo]

I think we can already load multiple models by putting wildcards in the load path.