Address failing numpydoc tests

[ukmo-ccbunney]

The following numpydoc errors are being generated and need addressing:

Numpydoc error codes

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1. GL08 The object does not have a docstring
2. PR01 Parameters {xxxx} not documented
3. RT01 No Returns section found
4. SS03 Summary does not end with a period
5. SS06 Summary should fit in a single line
6. SS05 Summary must start with infinitive verb, not third person (e.g. use "Generate" instead of "Generates")

These are currently being set to be ignored in the pyproject.toml