Do a better job of calculating the maximum halt velocity for the
stepper motors. The maximum cornering velocity is related to both the
maximum acceleration and the junction_deviation, so both should be in
the formula. Tests show that "math.sqrt(8. * self.junction_deviation
* self.max_accel)" very closely fits the maximum on cartesian robots.
This fixes potential "no next step" shutdowns that could occur on
some print moves.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>