Save derived bounds

[pp-mo]

Closes #6530

Note that the "proof" of the fix is to really compare
https://github.com/SciTools/iris/blob/487cc7a96cb0e48256b48f5f9c7124e9f0dc671c/lib/iris/tests/results/integration/netcdf/derived_bounds/TestBoundsFiles/test_save_primary_without_db.cdl
with
https://github.com/SciTools/iris/blob/487cc7a96cb0e48256b48f5f9c7124e9f0dc671c/lib/iris/tests/results/integration/netcdf/derived_bounds/TestBoundsFiles/test_save_primary_with_db.cdl
so .. in the second case, we get this magic extra line

---

Is this really ready to go ?

The main question there is, probably, more testing may be required - but can we think of any?
One thing to note : the new code is intentionally quite defensive
- i.e. things like "what if the named bounds-var doesn't exist", "what if the terms var doesn't exist"
- but a lot of that is probably not worth testing, since it may be impossible to get the saver to actually produce those cases

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.88%. Comparing base (e4191fb) to head (487cc7a).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/fileformats/netcdf/saver.py	94.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6540   +/-   ##
=======================================
  Coverage   89.87%   89.88%           
=======================================
  Files          90       90           
  Lines       23921    23937   +16     
  Branches     4461     4464    +3     
=======================================
+ Hits        21500    21515   +15     
  Misses       1669     1669           
- Partials      752      753    +1     

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