Relocate JPEG lossless data set

- to make room for other data sets under different conditions
- tweak data set README

Author: Enet4

benches/decoding_benchmark.rs

@@ -35,7 +35,7 @@ fn main() {
     }));
 
     c.bench_function("decode a 3072x2048 RGB Lossless JPEG", |b| b.iter(|| {
-        read_image(include_bytes!("../tests/reftest/images/lossless/jpeg_lossless_sel1-rgb.jpg"))
+        read_image(include_bytes!("../tests/reftest/images/lossless/1/jpeg_lossless_sel1-rgb.jpg"))
     }));
     c.final_summary();
 }

tests/reftest/images/lossless/LICENSE renamed to tests/reftest/images/lossless/1/LICENSE

@@ -0,0 +1,5 @@
+# JPEG Lossless data set 1
+
+The files starting with jpeg_lossless were taken from <https://github.com/rii-mango/JPEGLosslessDecoderJS>.
+The image data bytes were extracted from the DICOM files.
+The reference PNG files were created with the IJG library as distributed in the GDCM toolkit <https://github.com/malaterre/GDCM/tree/master/Utilities/gdcmjpeg>.