Merge dataless

[pp-mo]

Closes #6740

TBH this might have targetted "save_dataless" #6739 instead.
But now it can just wait till after that one

[codecov]

Codecov Report

❌ Patch coverage is 97.43590% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.32%. Comparing base (9c06cca) to head (1cb44e7).

Files with missing lines	Patch %	Lines
lib/iris/cube.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6741      +/-   ##
==========================================
+ Coverage   90.29%   90.32%   +0.02%     
==========================================
  Files          91       91              
  Lines       24656    24674      +18     
  Branches     4618     4623       +5     
==========================================
+ Hits        22264    22286      +22     
+ Misses       1620     1618       -2     
+ Partials      772      770       -2     

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[pp-mo]

Status Update Fri 2023-10-10

I'm currently using this branch to add some more extensive documentation, since I felt I needed a decent location to describe the properties of merging with dataless cubes.

Also, I want to at least refer to #6739

Hence put this in draft for a while...

[ukmo-ccbunney]

#6724 will allow for concatenation.

[pp-mo]

? I don't think so ? IIRC #6724 is only covering single-cube operations.
I thought we'd actually rejected merge + concatenate as too big for the current sprint.