Common metadata operations on split-attributes

[pp-mo]

As mentioned in #5410

This is about common-metadata operations : equal(compare) / difference / combine, as they apply to CubeMetadata, where the attributes properties are new-style "split" attribute dictionaries.

The #5410 PR provides that the new cube.attributes "CubeAttrsDict" can stand in for the legacy LimitedAttributeDicts where required, with a particular equivalence to the single dict form. Thus, everything using common metadata will still "work",i.e. not fail.

But @trexfeathers pointed out that we might want different behaviours, specific to split dictionaries.
For example, do we consider cube metadata equal if their atributes are CubeAttrsDict(globals={'x':1}) and CubeAttrsDict(locals={'x':1})? These are equivalent, when treated as single "combined" dictionaries, but that is probably not how we want the operation to work.

The basis of a possible solution is suggested here.

[pp-mo]

See pp-mo#77

I am happy now + ready to propose that this provides an appropriate solution.
To become an Iris PR once #5410 is in, as it is based on that code + will need rebasing.

[pp-mo]

Closed by #5538