Implemented PID_ADD_EXTRUSION_RATE term

Implemented the extrusion rate term on manage_heater(). 

The theoretical Kc calculated for PLA is 0.0078336. V^2*s/step. This was calculated from a calorific energy approach and we get that the extra power depends on voltage squared, and not on simple voltage. But, hopefully, in the boundaries on the problem, the error won't be that big.