pg35l_extruder: Place drive gear bearings based on drive gear size
pg35l_extruder: Place drive gear to exactly contact the filament

This change reduces the offset from 7mm to 6.375mm.

Also add some notes of measured hobbed pulley sizes.
pg35l_extruder: Simplify the filament funnel code

It's pretty odd when you change a drive gear OD and the funnel size
changes.  Re-work funnel code to be easier to understand and change.
pg35l_extruder: Rename filament_id to filament_dia

It's really an OD, not an ID, anyway.

Conflicts:

	pg35l_extruder.scad
pg35l_extruder: Separate the filament size and filament hole sizes

This is necessary when the drive gear is positioned based on the
filament size.  Using the true filament dia rather than the hole
dia moves the drive gear slightly closer to the filament.
pg35l_extruder: Split the difference and use average drive gear size
Merge pull request #4 from brandonheller/master

Improve pg35l positioning