Parametric linear E-steps calibration step cube for fine-tuning

Parametric step calibration piece for use with [Triffid's guide](http://reprap.org/wiki/Triffid_Hunter's_Calibration_Guide#E_Steps_Fine_Tuning).

I made this because my ring duct was large and made it impossible to see the surface finish of the 5mm step calibration piece without pausing the print and moving the nozzle away.