MyPy Type Checking in CI

[nigelmj]

Add static type checking to the ci pipeline using MyPy. Since the codebase already utilises the typing module, it would be useful to ensure type safety by integrating Mypy checks. Improvement to Ref #12 .

[nigelmj]

@suzanneEmbury As this issue is mainly an incremental improvement to the CI pipeline workflow, I can wait until the previous PR addressing Ref #12 has been merged before I create a new PR. If you would instead prefer a separate file for type checking I can create a PR by the end of this week.

[suzanneEmbury]

Just in case @nigelmj or anyone is interested in picking up this suggested change, PR #12 has been merged now.