Add pyehm plugin as optional tests

[sdhiscocks]

This enables us to flag plugin compatibility issues as well as enable tests for the Stone Soup plugin system

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ed5cf52) 93.35% compared to head (a3f2a95) 93.40%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #957 +/- ## ========================================== + Coverage 93.35% 93.40% +0.05% ========================================== Files 201 201 Lines 12642 12638 -4 Branches 2600 2600 ========================================== + Hits 11802 11805 +3 + Misses 597 591 -6 + Partials 243 242 -1 ``` | [Flag](https://app.codecov.io/gh/dstl/Stone-Soup/pull/957/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/dstl/Stone-Soup/pull/957/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl) | `66.61% <ø> (+<0.01%)` | :arrow_up: | | [unittests](https://app.codecov.io/gh/dstl/Stone-Soup/pull/957/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl) | `88.96% <ø> (+0.16%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl#carryforward-flags-in-the-pull-request-comment) to find out more.

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