Avoid banded non-contact region by reducing default stepsize.

Avoid banded non-contact region by reducing default stepsize.

See bug 18dd6d0a-3c45-4aba-80da-fa32ffaaf5db for example banded data.
(plot deflection without lines to see why I call it "banded").  Reducing
the stepsize reduces the jolt on the cantilever, and reducing the
points-per-step accordingly means we don't have to change the
card-clock rate.  It might be worth adding "target-clock-rate" option,
and letting the stepsize be determined automatically (as an
alternative to setting the stepsize explicitly) but we'll
put that off for now...