Added data for the SOI filtering gallery example.

This file will be needed for a new gallery example in iris (assuming it is accepted). The file is a modified (cleaned up) version of a variable in the file http://www.cgd.ucar.edu/cas/catalog/climind/SOI.nc, from http://www.cgd.ucar.edu/cas/catalog/climind/soi.html, which to my knowledge is in the public domain and has no restriction on usage