Initial 'strict' version

In this version only the terms were used that appear in the SDG policy text of the targets and indicators defined by the UN. At this point we have been aware of the SDSN Compiled list of keywords, and used them as inspiration. Rule of thumb was to use keyword-combination searches as much as possible rather than single-keyword searches, to be more precise rather than to yield large amounts of false positive papers. Also we did not use the inverse or 'NOT' operator, to prevent removing true positives from the result set. This version has not been reviewed by peers.