Start bumps with the cantilever _off_ the surface.

Start bumps with the cantilever _off_ the surface.

Photodiode sensitivity (and thus, bump slope) decreases as the
zero-deflection photodiode input moves away from zero Volts.  In order
to mitigate the effect of this on our calibration, we need to keep
track of the zero-deflection signal.  By starting our bump off the
surface, our bumps should always start and end in the zero-deflection
regime.

This will make analysis a bit more complicated though.  I'll get to
that next.