scan: Add stepper_tweaks (--no-stepper-tweaks) to UnfoldScanner.run()

scan: Add stepper_tweaks (--no-stepper-tweaks) to UnfoldScanner.run()

This is useful if you're not around to monitor a long run.  If the
laser is disrupted (e.g. via a bubble in the fluid cell), the
photodiode voltage can flatlign, even if you aren't off the surface.
A confused UnfoldScanner with *think* it's off the surface, and charge
blindly in, crushing the cantilever into itsy bits ;).

Laser disruption detection is a great reason to measure the total
photodiode voltage.  Unfortunately, that signal is not carried on the
cable between the MultiMode and the NanoScope.  I tried jumping it out
of the MultiMode by hand, but after a few weeks like that, the board
died.  I'm not sure it was related, but I think it's safer to just
disable stepper tweaks when you're not around to monitor the signal.