tune stepper function

In Mach step generators there may well be PID loops but they are hidden in the
external hardware. You can do the same with LinuxCNC by using the stepgens
position mode, but position mode is not as robust relative to host jitter as the
PID mode. In addition the PID stepgen control mode is easy to change to full
closed loop via external encoders.

Ultimately it would be nice to fix the stepgens position mode to make it more robust
and to have some of the more advanced tuning ability of the PID controlled stepgen
and perhaps hide the overhead requirements better, but that is a fair sized project

Peter thank you for increasing my understanding

There's stil so much I don't know, but I see a little gloaming in the distance ...