Netcdf separate loadsave

[pp-mo]

This "just" splits the iris.fileformats.netcdf module into separate sourcefiles for load + save code.
There's very little talk between the two, and this should makes it much easier for devs to find their way around.
Note that I haven't forced the split upon the user : All the public API is exported in the parent module, so everything there is as it was.

TODO: in draft while I test with Cirrus-Ci memory hacks, just to run the tests.

[pp-mo]

Status update :
doing sort-of OK
various tests to sort, largely Mock issues

[pp-mo]

Rebased following #4503 / #4809
Expecting various fixes still needed...

[pp-mo]

Rebased onto #4795 branch (which is passing tests), pro-tem, to get it testing with new GHA-based CI.
TODO: rebase again onto main branch, when #4795 is merged

[pp-mo]

Rebased onto main, following #4794

[pp-mo]

Hi @trexfeathers I think this is now ready.
No pressure, but ... I hope we can nail this before it needs rebasing again, because it has proved rather awkward for that.

[trexfeathers]

I think this looks fine, but there are some minor points I don't understand so hopefully you can help me with those.

[pp-mo]

Brilliant thanks @jamesp .
Busy elsewhere but will respond in a bit ...

[pp-mo]

## Context : things this can be used for ?
In addition to just "fixing data a bit" on either load or save,
I think this might possibly help with a number of awkward outstanding desired features relating to netcdf :

• load chunks control : More control over lazy data creation (chunking) #3333
• precise formatting round-trips : Round-trip netcdf data #4215
• lazy saving : Support lazy saving #4190
• output "append" mode : Add functionality to allow appending cubes to existing netcdf file #565

UPDATE: oops, put this on the wrong issue !

[pp-mo]

Thanks @trexfeathers .
I think I finally have it working now (after several tries at fixing the import problem!)
Can you please re-review ?

[trexfeathers]

Thanks for the responses @pp-mo, and sorry for the delay.

Please could you add a What's New entry under the Internal heading, as this change is important to flag for developers.

Please also shout if merging this now is going to cause any problems with the Split cube-attrs project. Perhaps it's worth the problems 😅

[pp-mo]

Please also shout if merging this now is going to cause any problems with the Split cube-attrs project. Perhaps it's worth the problems sweat_smile

Good thought, but should be nothing serious I think -- this has only moved things, not reorganised.

[trexfeathers]

I've been re-running some missed benchmarks. FWIW you would have seen this (predictable I suppose!):

       before           after         ratio
     [e96a0536]       [d8f8dac7]
-         646±5μs          130±1μs     0.20  import_iris.Iris.time_fileformats_netcdf