Create average-link-strength.txt

This file has the average link strength S as computed by `ludescher.R` at 10-day intervals, starting from day 730 and going until day 12040, where day 1 is the first of January 1948. For an explanation see [Part 4](http://johncarlosbaez.wordpress.com/2014/07/08/el-nino-project-part-4/) of the El Niño Project series.