vuMPS

[alewis]

I'm now just one or two merges away from being able to start committing the example vuMPS code to the experimental_ncon branch, from which point on the latter will be a self-contained prototype.

The example would, however, be one to a few thousand lines of pretty nontrivial code that doesn't really divide into logically independent fragments. I'm not sure how to break it up into digestible PRs.

I think what I'll do is commit the example into its own separate repository. Then maybe Martin could have a look and we can discuss how to break it up.