Filament drive fan

Update: it turned out this didn't actually work. What finally fixed the problem (only just recently!) was running the steppers with 24V, which also enabled them to run at their specified current (and more quietly/better sounding). I probably also significantly reduced voltage fluctuations in the process.

My bot has had a real problem with the filament drive wheel slipping for a long time now, which would occasionally result in stripping, especially on long prints, and thus lots of frustration