gdal/autotest/proj_grids

* ``conus`` is the "official" NADCON4 conus NAD27->NAD84 file in CTable2 format
* ``egm96_15_extract.gtx`` is an extract generated with ``gdal_translate /usr/share/proj/egm96_15.gtx ../autotest/proj_grids/egm96_15_extract.gtx -srcwin 234 233 3 3``