Provide "combine_cubes" function to integrate new merge/concatenate/load facilities

[pp-mo]

See as proposed pp-mo#82, which targets the preliminary refactor in #6199

The intention is to merge in several factors in one place :

• make the old LoadPolicy an inheritor of a CombinePolicy, for more consistent naming ...
  • and incorporate a 'merge_unique' keyword ...
  • as mentioned here
• include the new equalise_cubes operation as a programmable pre-merge/concatenate step within the combine operation,
  • as mentioned here

I'm making a separate issue of this from pp-mo#82, just to be clear + label that I think we should aim to tidy all this up and get it into Iris v3.12