Add Jedi string method to datetime

[DanStoneMO]

PR Summary

Sci/Tech Reviewer:
Code Reviewer: mo-lucy-gordon

This is a small PR adding a new method to the datetime object, allowing it to be written as a ISO datetime string compatible with JEDI systems. The process of converting between LFRic and JEDI compatible datetime strings is currently hard-coded in lfric-jedi, which would be bad for maintainability down the line.

The new method is largely a copy of the current to_string one, just adjusting the characters used in the output string.

linked PRs

• linked MetOffice/lfric-jedi 1321

Code Quality Checklist

• I have performed a self-review of my own code
• My code follows the project's style guidelines
• Comments have been included that aid understanding and enhance the readability of the code
• My changes generate no new warnings
• All automated checks in the CI pipeline have completed successfully

Testing

• I have tested this change locally, using the LFRic Apps rose-stem suite
• If any tests fail (rose-stem or CI) the reason is understood and acceptable (e.g. kgo changes)
• I have added tests to cover new functionality as appropriate (e.g. system tests, unit tests, etc.)
• Any new tests have been assigned an appropriate amount of compute resource and have been allocated to an appropriate testing group (i.e. the developer tests are for jobs which use a small amount of compute resource and complete in a matter of minutes)

Test results, via mo-bundle's mobbalt opt: https://cylchub/services/cylc-review/taskjobs/daniel.stone/?suite=mob-jedi-time-convert-mobbalt-2

Code Review

• All dependencies have been resolved
• Related Issues have been properly linked and addressed
• CLA compliance has been confirmed
• Code quality standards have been met
• Tests are adequate and have passed
• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable