Implement RSet Support for NITF Driver

[warmerdam]

It is desirable that the NITF driver support utilizing reduced resolution datasets in "RSet" format. These are a series of NITF files with the same basename as the primary file but with reduced resolution. The reduced resolution files have extensions in the range r1 to r5.

eg.

warmerda@gdal64[77]% ls -l yel*
-rw-r--r-- 1 warmerda warmerda 32069189 2010-03-04 13:58 yellowstone_2.1.ntf
-rw-r--r-- 1 warmerda warmerda  8015531 2010-03-04 13:54 yellowstone_2.1.r1
-rw-r--r-- 1 warmerda warmerda  2003348 2010-03-04 13:56 yellowstone_2.1.r2
-rw-r--r-- 1 warmerda warmerda   500309 2010-03-04 13:56 yellowstone_2.1.r3
-rw-r--r-- 1 warmerda warmerda   125774 2010-03-04 13:56 yellowstone_2.1.r4
-rw-r--r-- 1 warmerda warmerda    31835 2010-03-04 13:56 yellowstone_2.1.r5

When rsets are utilized as overviews they should be considered read-only. There is no need for the driver to be able to create rsets in BuildOverview().

Other notes:

• If there are overviews in other formats, they should likely take precidence over the RSets.
• An exception is that "built in" overviews in jpeg2000 data streams should not take precidence over the rsets since accessing them tends to be slow.
• A call to BuildOverviews() should likely use the normal default (.ovr or .rrd) logic after which the rset overviews should be ignored.
• It is imperative that the rsets work with nitf files using jpeg2000 compression internally.