Pressure Advance

I did some more testing with the pressure advance algorithm. Overall it works really great! The quality of the printed objects is far better and with a lot less leaky spots.

I did find two problems. For one, when the pressure advance is pressing more filament into the nozzle, it is basically the same as an unretract, so it probably should use the unretract_speed as well.

Secondly, when you are modifying an existing unretraction in the gcode, you added the original value of the retraction to the return value of extruder->extrude(), which is unnecessary, because the extrude function basically does that already, which amounts to that you are unretracting two times the retraction length plus the pressure advance length.