gdal_merge.py : add support for output of nodata value

[etiennesky]

The gdal_merge.py does not set the nodata value in the bands of the output file.

I have added a -a_nodata option (as in the other tools gdalwarp and gdal_translate), which adds the specified nodata value to the output file (regardless of the -a value).

+ if a_nodata is not None: + t_band.SetNoDataValue?(a_nodata)

Tested on GTIFF files only. I'd be glad to test other files types if necessary.

[etiennesky]

svn diff of file swig/python/scripts/gdal_merge.py

[rouault]

I've commited a more compact version of the proposed patch:

r22325 /trunk/gdal/ (apps/gdal_utilities.dox swig/python/scripts/gdal_merge.py): gdal_merge.py : add a -a_nodata option (#3981)