Re-adding scipy optimize to on-demand imports

[chummels]

Recently, when the on-demand imports were being cleaned up in #c501b8c, scipy.optimize was removed as an alias. It turns out that this broke Trident, so I'd like to add it back in.

[neutrinoceros]

Seems reasonable. I suggest adding a comment there so we know why it's still here, though in the long run I'd suggest trident could use it's own on-demand module.

[chummels]

Well, I considered adding it to Trident, which used to be part of yt several years back, which may have been the reason that this code was present. But it's a lot more code to add all the on-demand stuff to Trident rather than just adding these 4 lines to yt. I'll add a small comment.

[neutrinoceros]

I understand that. If you're interested I would gladly volunteer to do the work on Trident too.