`min_distance` doesn't work with 3D tracking where `dz` not constant

[freemansw1]

Reported to me by Yishi Hu from OU. When you use min_distance in feature detection with a non-constant dz, it fails as the coordinate interpolation takes place after min_distance is called. That means min_distance doesn't have any vertical information. This should be able to be solved by reversing the order of the two calls.

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