Ditch weights, use scale instead

I was struggling, unnecessarily, to use weight ratios to do complex
calculations to get different movement functions to have more or less
influence. It seems like I can get the same thing just by scaling the
results of the different movement functions and then capping the
aggregated results.
Fix world tests

Tested that world.movements is created and copied properly; I was only
testing that world fields were set when changing boids, not that other
fields were preserved.