String stash code silently ignored on pp-save

[rcomer]

🐛 Bug Report

Before saving to pp-file, a user may have reason to modify or add a stash code. If the code is added in the string form, it is silently ignored.

How To Reproduce

Steps to reproduce the behaviour:

1. Run code

import iris

fname = iris.sample_data_path('air_temp.pp')
cube = iris.load_cube(fname)

cube.attributes['STASH'] = 'm01s34i001'

iris.save(cube, 'test.pp')

2. Inspect the file test.pp.

3. The stash code in the file is zero.

Expected behaviour

Either the stash code in the file is 34001 or I get a type error because I should have used a STASH object instead of a string.

Screenshots

Environment

• OS & Version: RHEL9 🎉
• Iris Version: 3.11

Additional context

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[rcomer]

I would be happy to write a PR for this, but does anyone have an opinion what the behaviour should be? Should it "just work" via from_msi? Or should it error? Or just warn that the attribute is wrong type and therefore ignored?