split-attrs: cube attributes

[pp-mo]

Note: #5040 now to implement this

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"cube.attributes" to become a wrapper object,
providing read+write accesses ~same as existing (combined local+global), plus access to local + global settings specifically, when required.

At this point, we expect all existing tests to pass unmodified

• maybe just a few odd exceptions ?

(N.B. have come code for this prototyped already)