Calculates a Sholl analysis on tree ** intree** counting
the number of intersections of the tree with concentric spheres
of increasing diameter values given by

Equations for intersection between line segments and spheres are from Paul Bourke 1992:

http://local.wasp.uwa.edu.au/~pbourke/geometry/sphereline/

Example:

>> sholl_tree (sample2_tree, 20, '-s')

The calculation happens in 3D:

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