Support for Sandia National Laboratories GFF data format

[pvachon]

This patch provides rudimentary support for the Sandia National Laboratories GSat File Format image data. This data is generally complex, however, there is support for images with 1 byte/pixel (and this case is accounted for, though untested due to a lack of data).

This format is read-only, and very limited metadata support exists at present. The next goal is to include more detailed polarimetric information, as well as date/time information and instrument parameters. Unfortunately, no data on the format has been made available to me other than a Matlab program to parse the data, so support for much beyond that is a bit hectic.

Sample data for this can be found at ​http://sandia.gov/RADAR/sar-data.html. The format was tested using gdal_translate (to translate it to a GeoTIFF image) as well as OpenEV to display both the resulting GeoTIFF and the image itself.

[pvachon]

The patch itself to support GFF format, against HEAD.

[warmerdam]

Patch applied and some manual reformatting done.

A brief help topic should really be added to gdal/frmt_various.html and referenced from the formats_list.html.