review coverage of code

[orbeckst]

Once PR #54 is merged (and before making the 0.2.0 release #65 ) we need to check code coverage.

• ensure that the integration and unit tests properly aggregate coverage
• remove code that is not used and not tested
• raise issues for testing essential code

[orbeckst]

• The workflow creates coverage.yaml in two different steps: IMDClient tests and package utility tests; these would need to be merged (which is possible with coverage/pytest-cov) or sent separately to codecov.
• The simulation engine tests do not even collect coverage information (nor use codecov). Should be added.
• We should update codecov-action to v5 (see https://github.com/codecov/codecov-action for necessary changes).

Note: codecov does not run in scheduled CI.

[orbeckst]

I reviewed what needs to be done. @amruthesht @hcho38 @ljwoods2 please feel free to tackle any of the sub-issues.

I'll leave this issue open until the sub-issues have been addressed.