Use stepper.single_step instead of stepper.step_relative in AFM methods.

Use stepper.single_step instead of stepper.step_relative in AFM methods.

This avoids problems with steppers in full-step mode always rounding
the asked-for half step back to the current position.  The drawback to
the new approach is that we loose backlash protection when moving away
from the surface, but the surface fitting condition ...?