numerically stable radius penalty

[tylerflex]

previously, I smoothed the radius of curvature calculation by adding 1e-2 to the denominator. This avoided it going to infinity for infinitely straight edges. However, when the actual radius of curvature was close to 1e-2 it gave inaccurate results. Instead, I made the penalty function a numerically stable version for x > 0, which solves the issue more generally.

old

penalty(x) = 1/(1 + exp(x))

new

penalty(x)= exp(-x)/(1 + exp(-x))