Add section in wiki describing where one can get test images

[boxerab]

Besides the data for the test suite.....

For example, here are two sources for geospatial images (very large tiled images)

You can download Sentinel-2 products at :
https://scihub.copernicus.eu/s2/#/home
up to 10980x10980 images, 12 bit, with 2048x2048 tiles

For Sentinel2, you can log in as guest:guest,

Type "S2A_" in the search area and you'll see lots of products with names like S2A_OPER_PRD_MSIL1C_PDMC_xxxx . They contain .jp2 files. You'll find the .jp2 in
the GRANULE/_/IMG_DATA subdirectories

Pleides products :
http://www.geo-airbusds.com/en/23-sample-imagery
Bigger dimension than S-2. Also 12 bit, with 2048x2048 tiles

Search Optical Imagery/Pleides/ JPEG 2000