Structure generation of pure elements

As we discussed, the conc extrema have been a little weird.

This fixes that, at least for my case. So the idea is that we iterate through the templates in order, and only accept it once we have gotten a valid concentration.

Also, there was an issue where we weren't respecting the fact that settings.size is user specified, and should never be changed by the settings class.

Also, snuck a cubicness function in there, as it got added and then no longer was necessary while doing this, but I figured it might be useful at some point.

Also closes #258 (closed)