Add time-processed multi-var plotting

[ukmo-huw-lewis]

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• Documentation has been updated to reflect change.
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[github-actions]

[ukmo-huw-lewis]

Updated mult-variable recipe, and uncommented commented-out workflow option to make SPATIAL_MULTI_FIELD_METHOD active for multi-variable plots.

Approach from #1986 adopted in recipe, but with extra checks for cell_methods using filters.filter_multiple_cubes - else can read in multiple instances of same cube names.

Example outputs:

Illustration 'SEQ' method still working after changes:

Illustration processing 48hr inputs with METHOD="MEAN" via cset bake on command line:

[note title here picks up '2 cases' rather than start/end date of the case run - ideally would need to unpick this further, but risk reopening aspects of #1954 so propose to defer to another issue/PR in slower time]

Illustration processing inputs with METHOD="MAX" via cset bake on command line:

Likely not best illustrations of use-case where time analysis useful for enhancing evaluation insights, but demonstrates functionality works, and has circumvented issues of prohibitive memory/speed requirements when initially read.read_cubes got everything from input files.

Ready for review.