Clock stop event

[dwastberg]

Add an onStop Event to the Clock which is called when the clock reaches its stopTime.

[cesium-concierge]

Thank you so much for the pull request @dwastberg! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

The Pull Request Guidelines is a handy reference for making sure your PR gets accepted quickly, so make sure to skim that.

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[dwastberg]

CLA has been sent

[pjcozzi]

Thanks again for the contribution, @dwastberg. We received your CLA.

[hpinkos]

Thanks @dwastberg! This PR looks fine to me. Can you please explain a scenario where this event is useful?

[dwastberg]

I'm using the clock to control a looping simulation and at the end of each loop I want to clean up some objects placed in the scene and make some changes to my GUI.

[hpinkos]

remove this whitespace

[hpinkos]

Thanks @dwastberg! Just that one comment. And please add a note under the Additions section of CHANGES.md

[dwastberg]

Just wondering if there is anything else needed to merger this?

[hpinkos]

Thanks for the reminder @dwastberg! We don't get notifications when you add more commits, so it's a good idea to make a comment to let us know when a PR is ready for another look.

This all looks good to me.
Does anyone else have feedback? If not, I'll merge this by the end of the day

[hpinkos]

Thanks again @dwastberg!