ENH: support general searchable spheroid (issue #1901)

- a sphere/spheroid can be specified as a single radius or three radii.
  If all three values happen to be identical, they are collapsed to a
  single value. Examples,

      radius 2;
      radius (2 2 2);
      radius (2 3 4);
      radius (2 2 4);

  The search for nearest point on an ellipse or ellipsoid follows the
  description given by Geometric Tools (David Eberly), which also
  include some pseudo code. The content is CC-BY 4.0

  In the search algorithm, symmetry is exploited and the searching is
  confined to the first (+x,+y,+z) octant, and the radii are ordered
  from largest to smallest.

  Searching is optimized for sphere, prolate and oblate spheroids.