PP derived bounds

[pp-mo]

Separate PR direct to main
cf HGWright#4

(
So, this is basically a replacement for
#6350
which itself was a replacement for the original
#6190
)

[codecov]

Codecov Report

❌ Patch coverage is 92.15686% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.83%. Comparing base (7927212) to head (0b370aa).
⚠️ Report is 137 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/fileformats/cf.py	91.48%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6481      +/-   ##
==========================================
+ Coverage   89.82%   89.83%   +0.01%     
==========================================
  Files          90       90              
  Lines       23782    23828      +46     
  Branches     4427     4448      +21     
==========================================
+ Hits        21361    21407      +46     
+ Misses       1672     1670       -2     
- Partials      749      751       +2     

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[HGWright]

Thanks Patrick, it was a big collaborative effort. This is a big part of #3678.