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[eldobbins]

This PR addresses 2 related issues: previewing lessons on a local machine (in this case MacOS Mohave) and running the python-based lesson checking scripts included in https://github.com/carpentries/lesson-example

• Previewing. Previewing lessons on a local machine requires a Ruby environment with gems like Jekyll. I accomplished this via Anaconda. The resulting environment is described in environment_mac.yml
• Lesson checking. This required updating the Makefile and copying in scripts and some _include files from the lesson-example. There are many more that could be copied, but I didn't want to alter anything that was already working, so I kept it to a minimum.

make lesson-check runs but does not finish successfully. I hope to submit future PRs that correct the errors.

[DamienIrving]

Thanks, @eldobbins.

I've requested a review from @fmichonneau because if I'm totally honest, I know very little about how the build works. Up until now I've just been pushing changes to GitHub and then waiting to see if the changes appear as I thought they would at the github.io site, which obviously isn't a sustainable development process going forward once other people want to make contributions.