declare knee if Tmx negative

[arvkevi]

This change will declare a knee at the local maxima in the distance curve, if the threshold, self.Tmx is negative.

A negative threshold was pointed out in #13 -- With only 5 data points, the difference between the normalized x points, 0.20, was high relative to the maxima on the difference curve 0.18.

[arvkevi]

It now finds the knee correctly here, but I don't know if this change will exhibit expected behavior in other, similar situations, so I'm going to wait to merge until further testing can be performed.

x = [3,4,5,6,7,8]
y = [1.4699,1.4149,1.3868,1.3594,1.3411,1.3265]

kneedle = KneeLocator(x, y, S=1.0, curve='convex', direction='decreasing')
kneedle.Tmx
array([-0.01645746])

kneedle.knee
4