Problem 636. Spherical radius given four points

Determine the radius of a sphere provided four non-coplanar points on the surface of the sphere.

 pts = [x1 y1 z1;
        x2 y2 z2;
        x3 y3 z3;
        x4 y4 z4]

Hint: See http://steve.hollasch.net/cgindex/geometry/sphere4pts.html

Example:

 pts = [1 0 0;
        0 1 0;
        0 0 1;
       -1 0 0]
 r=1