AI Fireman looses pressure when starting on steep gradients

When a steam train is required to start on a steep gradient after boiler heat has been built up, the AI fireman does not start fueling the fire soon enough to prevent steam pressure from dropping too quickly.

This bug fix adds logic to the AI fireman to allow for fuel to be added to the fire when on steep gradients and the steam heat in the current logic would normally prevent this from happening. (ie a crude form of "look ahead").