This directory contains stuff for the composite custom tracks.  A composite custom track is a 
directory full of binary files which lives predominantly on the user's machine.  It is set up so 
that a minimum amount of information for a particular genomic view needs to be sent.

The overall structure of the directory is:

I. Table of contents and metadata file.
II. Data file for each zoom level of each subtrack.
III. Index file for each zoom level of each subtrack.

The table of contents and metadata are small enough to fit into memory and to be sent in most 
cases in a single 16k request.

The data files in most cases are straightforward binary translations of such things as bed, psl,
and wiggle files.

The index files are based on a one-dimensional R-tree created using a simple sort and partition 
strategy.  

