Detection problems in importers

[klakhov]

Most of the time (some tests for it here and here) we expect importers to work successfully with empty datasets. But even if we can export such datasets, in some formats, we cant detect them "back" with detect_dataset method. That happens because detector methods expect annotations not to be empty and throw an error like:

"'/gt.txt' must be a ICDAR-like annotation file"

List of formats:

• CamVid (detector)
• Imagenet (detector)
• Icdar (IcdarWordRecognitionImporter, IcdarTextLocalizationImporter - some problem with path, IcdarTextSegmentationImporter - .txt file required per each .bmp)

The solution here may be correcting the detection code so it can accept empty files.