balancing

this balances / fixes the growth/consumption interactions It works quite nicely now, there are 2 big tipping points, one is the collapse of the grasslands, the other (later one) is the collapse of the forests. The latter needs to be helped along a bit by "global thresholds" that kick in once the tree cover is sufficiently low. That's mainly to speed things up/don't make the player wait for too long - but it also could be interpreted to model some precipitation/water level effects that affect the whole ecosystem once tree cover is too low. These are not modeled in detail here, to keep it simple.