Splitattrs ncload

[pp-mo]

Update: Monday 3rd July
still waiting on #5040, but looking generally OK and "close"
Basically, what truly belongs here is the last 2 commits only

• note that the actual changes made in the main "Distinguish local+global attributes in netcdf loads." commit seem really tiny,
  but that's just because a handy API is already implemented in Implement split cube attributes. #5040 : it only remains to assign global and local attrs to the cube in "slightly different" ways.

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Addresses #4984
WIP for the present,

1. just to see which existing tests may fail.
2. based on Implement split cube attributes. #5040, so wait on that + may need rebasing

It will be a good idea to adopt #4960 here, which should test legacy operation more thoroughly.
But #4960 still has some outstanding work to complete (review asks for more tests).

[pp-mo]

Replaced