Auto-METAL collection

After receiving feedback stating that there wasn't enough time to collect all the metal in the given time frame between waves, I made it such that all the metal are automatically collected after a wave.

That said, I don't think this feature is necessary for the initial part of the game, and is more suited as an upgrade - which is why I've made the feature easily toggleable. I suppose before long we'll have to work on the upgrade system as well.