Add "kira tree" support -- tree arcs, with crosswise tick-marks
at center-of-mass points.  Tickmarks lie in screen plane, with
length of <tickscale> times true separation.

Re-extract whenever kira_set changes something.
Add particle tracking.
Compute ring sizes correctly, and draw rounder rings.

Add ring markers for non-isolated stars.  Not finished yet, but seems usable.
No GUI yet, but new commands:
   kiractl nodes  {on|off|roots}
   kiractl rings  {on|off|roots}
   kiractl ringsize {sep|semi}   (ring size from curr. separation or semimajor axis)
   kiractl scale  <scalefactor>
   kiractl span   <minpixels> <maxpixels>   (allowed range of ring radii)

Change encoding of nclump: n for leaves, -n for center-of-mass nodes,
   so colormap can make them distinguishable.

Bug: after kiractl command, must change timestep before it takes effect.
Bug: ring markers aren't colored yet for some reason.

Glue code to read starlab (kira) files and interpolate particle positions
into partiview internal form.