returns the s-coordinate of the point of closest approach of
the RAYS to the origin x=y=z=0. The length of the first dimension
of RAYS may be either 3, 5, or 6; this first dimension will not
be present in the result.
The s-coordinate represents distance along the ray, increasing in
the direction the ray moves. The 5 and 6 component ray coordinates
include a reference point (x,y,z) on the ray; s=0 at that point.
For the 3 component ray coordinate, get_s0 always returns 0.
interpreted function, defined at i/rays.i line 223