Fan deflector now has 4 screw holes

Added 2 more screw holes to the fan deflector. This allows 4 mounting
screws to be used to hold the fan to the heatsink, to avoid distorting
the fan, which can make it noisy.
Added modified version of iambury's extruder gears

Added a version of iamburny's double helix extruder gears, modified to
make the holes for the motor shaft and the hex bolt customisable. Before
I did this and increased the sizes, I found they were too tight to fit
on the motor and the bolt.