Test examples periodically and on commits to mesa main

[EwoutH]

I think the examples should be tested in the following cases:

• when we change the examples here (against both the latest mesa stable version and mesa main)
• when a commit is pushed to mesa main
• periodically (every week) (against mesa main and stable)

We can probably recycle some code from projectmesa/mesa#1956. @rht since you wrote that, would you like to do this?

[rht]

when we change the examples here (against both the latest mesa stable version and mesa main)

The CI in projectmesa/mesa-examples already includes mesa-examples: #82.

when a commit is pushed to mesa main

Already covered by projectmesa/mesa#1956

periodically (every week) (against mesa main and stable)

I find this redundant to the second checklist item.