Adding github code space development environment

[shimwell]

I'm keen to make a PR that adds a cloud (github code space) development environment for the openmc repository.

I've added a PR for code space environment for users of the example openmc-notebooks repo openmc-dev/openmc-notebooks#2

This is for people wanting to spin up a cloud environment to run the notebooks in, with openmc and nuclear data preinstalled

However I think we could benefit from a developer code space that is slightly different for this openmc repo. This could help devs make, test and review branches.

For this to be fully effective it would be necessary to change some code space settings in the github org.

I would be keen to make a PR to explore this idea if people are interested.