Address failing mypy tests

[ukmo-ccbunney]

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

### Error code
- [ ] `arg-type`
- [ ]  `assignment`
- [ ]  `attr-defined`
- [ ]  `func-returns-value`
- [ ]  `misc`
- [ ]  `no-untyped-call`
- [ ]  `no-untyped-def`
- [ ]  `operator`
- [ ]  `redundant-expr`  Note: this one should be explicitly enabled in `enable_error_code`
- [ ]  `unreachable`
- [ ]  `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.