Quick_Pulse_Distance only works with 2.5, not 1.25

[nathan-mars]

Quick_Pulse_Distance register only works with 2.5, not 1.25. This is not a problem operationally, since we only plan to use the 2.5μm/pulse faster motors with fast retract in experiments, but it is strange that a 1.25μm/pulse standard motor can't be used. If the 2.5-only limitation is expected, I'm not sure why there's a register to set the pulse distance.

What we see: set all parameters using 2.5 for Motor1_Quick_Pulse_Distance (or Motor2), motor fast retracts as expected. Change to 1.25, changing no other parameters, and no movement is seen. The driver seems to accept the 1.25, I don't remember seeing an error when the register is set.