Fix string formatting for numpy array scalars

[dopplershift]

Unlike normal arrays, the scalars do not respond to set_printoptions, so instead normal formatting needs to be used. (See numpy/numpy#11048 for ongoing confusion about this.) Also, for HTML formatting, they are flagged as iterable even though they should be formatted like single values.

• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

[jules-ch]

Looks good !