move definition of POINTMAP to own file

1. This commit finalizes the move of the POINTMAP variable from responses.js to
   its own source file, pointmap.js (where it is less obtrusive).  The code in
   pointmap.js defines a global namespace object, hmslincs, and defines the
   POINTMAP variable as a member of it.  (See code comments.)

2. The pointmap.js script is loaded by the responses/index.html page, right
   before it loads the responses.js script.

3. The pointmap.js script is automatically generated by the deploy script
   (through do_scatterplot.sh).