More Axes in Marlin

I just realized that Marlin supports 6 non-extruder axes since about a year ago, and in the latest (not released) version it supports up to 9!

Previously I had contemplated maybe faking extra axes with a mixing extruder, but this is not good for a few reasons. One issue, even with a single rotary axis driven by E, is that there is no concept of homing for E. The extra non-extruder axes appear to fully support homing, with endstops for each axis.

Maybe it’s time for a burrito robot with one axis for each ingredient.