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

Note:

Since we now have the abililty to save+load dataless cubes, this also updates make_gridcube to be dataless

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.38%. Comparing base (9c06cca) to head (58aca88).
⚠️ Report is 12 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6741      +/-   ##
==========================================
+ Coverage   90.29%   90.38%   +0.08%     
==========================================
  Files          91       91              
  Lines       24656    24743      +87     
  Branches     4618     4638      +20     
==========================================
+ Hits        22264    22364     +100     
+ Misses       1620     1608      -12     
+ Partials      772      771       -1     

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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]

Looks good Patrick 👍🏼
Just a few comments.

[pp-mo]

Thanks for latest @stephenworsley.
Hopefully feba264 addresses those.

[stephenworsley]

Looks good!

[pp-mo]

Brilliant, thanks @stephenworsley
I was so keen to get this in, for the docs improvements.
(and I really should have submitted that separately!)