bibtools/apj-issnmap.txt: add another JGR ISSN
bibtools/apj-issnmap.txt: add another JGR ISSN

This is from the information provided for 10.1029/93JE00981. It is apparently
published in the Planets sub-journal, but I don't really understand how the
sub-journals are supposed to work. The recommended citation reads only
"J. Geophys. Res.", for what that's worth.

pkgw/bibtools,pkgw/bibtools