Improved non-zero coordinate finder

Hi Timothy,
Found your repo via reddit and enjoyed your paper, and thought I'd contribute to your code. A small PR to get the ball rolling..

* Improved ``find_3D_object_voxel_list function`` -- much simpler, and should offer a performance boost.
* Added this function to ``kline_vessel`` function.

Tests give same results with updated code.

Cheers,
Adrian