Address failing mypy tests

[ukmo-ccbunney]

The following typing errors are being generated by mypy and need addressing in the code:

Error code

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1. arg-type
2. assignment
3. attr-defined
4. func-returns-value
5. misc
6. no-untyped-call
7. no-untyped-def
8. operator
9. redundant-expr Note: this one should be explicitly enabled in enable_error_code
10. unreachable
11. var-annotated

Currently, failing type checks are added to the disable_error_code lust in pyproject.toml

Note, the cf_units/_udunits2_parser is being ignored and is out-of-scope for this Issue.