Retrieve examples/mechanisms files from `neurodamus-models` repository

[darshanmandge]

I can tackle this in a seperate issue if that is fine with you

Yes, that plan sounds good. Probably you just do git clone neurodamus-models repo before executing this line in tox.
I also see that BlueCellLab has 2 places for mod files, examples/mechanisms and tests/mechanisms. It would be good to use one location with sub folders like neocortex, hippocampus, etc.

I don't have other comments for this PR. I will let @jamesgking comment.

Originally posted by @WeinaJi in #67 (comment)

This will ensure the mechanisms are the latest every time.