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. The base file has the extension r0.

eg.

1585469225 Jan 30  2007 03SEP24175829-P1BS-005597228030_01_P001.ntf.r0
 396384276 Feb  7 10:37 03sep24175829-p1bs-005597228030_01_p001.ntf.r1
  99112980 Feb  7 10:37 03sep24175829-p1bs-005597228030_01_p001.ntf.r2
  24795156 Feb  7 10:37 03sep24175829-p1bs-005597228030_01_p001.ntf.r3
   6215700 Feb  7 10:37 03sep24175829-p1bs-005597228030_01_p001.ntf.r4
   1570836 Feb  7 10:37 03sep24175829-p1bs-005597228030_01_p001.ntf.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.

[warmerdam]

My intention is to wrap the overviews in an in-memory VRT. As a first step I have implemented support for VRTs to hold overviews in trunk (r19011).

[warmerdam]

I have integrated support for rsets in trunk's nitfdataset (r19018) and backported to the 1.6-esri branch (r19019).

I am leaving this ticket open to develop something for this in the test suite. But the feature is not ready for other testing.

[warmerdam]

Minimal testing incorporated in trunk (r19020).