comment to document a private method
bugfix 4845: image reader can now read from urls provised via the input table
- images are downloaded to the tmp directory and openened via scifio
- file-urls are converted back to absolute paths
- option "use complete file path as row key" has no effect when read from an url

other little modifications: unused static fields removed, just throw warning message but continue reading images, when an image file doesn't exist
Image reader node description modified (added hint that images can be read from urls).