Compute coverage and visibility statistics for a collection of isovist rays.

Input parameters:

1. Ry → Isovist rays. Use the Rays output of an Isovist 2D or Isovist 3D component.

Output parameters:

1. Cv → Fraction of the original sampling dome occupied by rays that hit an obstacle.
2. Ht → Number of rays that hit an obstacle.
3. D↓ → Smallest visible distance among hit rays, or zero if there were no hits.
4. D• → Average visible distance among hit rays, or zero if there were no hits.
5. D↑ → Largest visible distance among hit rays, or zero if there were no hits.
6. Sd → Standard deviation of visible distance among hit rays, or zero if there were no hits.