use polyholes
Back to mini rod lengths.
Making things more parametric:

* smooth_rod_separation
* smooth_rod_radius
* smooth_rod_bearing
* idler_bearing

(e.g. should work for lm10uu on 10mm rod using 16 tooth pulleys / 624 idler)

* rod_radius and rod_length are the values that should be able to go
  directly into smoothieware delta config as arm_radius and arm_length

tower_radius is derived from other parameters, notably rod_radius and
the carriage and platform offsets.