Adding code space env

[shimwell]

This PR adds a dockerfile and user environment for anyone wanting to spin up these examples in github codespaces.

I've demonstrated this to @pshriwise as it provides an easy back up option for the live demo at the IAEA conference.

For this to be most effective code spaces would ideally be enabled on this github org but perhaps that is a step to far

[RemDelaporteMathurin]

Wow, thanks @shimwell for showing me this.

I ran this repo in a codespace, it took a little while to build. But then everything was like developping locally. You can even open the codespace locally (in VSC) which is in fact 100% local development!

I highly recommend making the openmc repo codespaces friendly and even consider it for my current projects!

It appears that the potential long building stage can be solved by pre-building the image (Am I right @shimwell ?)

[shimwell]

Yep prebuilds would bring down the time to launch and can be setup in the repo settings. Most of the current install time is due to nuclear data downloading, so it could also be shorter by dynamically getting the data using on the fly data downloading if we want https://pypi.org/project/openmc-data-downloader/