Wai.annotations

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- Added ability to provide a source iterable when using `iterate_files`.
- `ConversionPipelineBuilder` now works with any combination of sources, processors and sinks.
- Data elements now are only referred to by their base filename.

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- Added the layer-segments image-segmentation format.
- Added the `od-to-is` cross-domain converter.
- Bug fixes.

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- Fixed bug in calculating output directory from command-line option.
- Added `strip-annotations` ISP.
- Stopped COCO/ROI/VGG image object-detection formats from writing negative annotations.
- Fixed bug in coercions where negatives instances weren't being handled correctly.
- Added `--dense` option to `to-tf-od` which encodes masks in the dense numerical format.

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- Refactor of the internal structure to aid in debugging.
- All input/output formats now end in the short-code of their domain.

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- Fixed bug where `--category-output-file` was being written to an arbitrary location.

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- Added --category-output-file option to to-coco format for writing the categories to a simple
comma-separated text file.
- Added the subdir image classification format.
- Added the ADAMS image classification format.
- Refactored label-mapping, image format conversion and label filtering into their own
ISPs (were options on object detection readers/writers).
- Better error messages when domains are incorrectly matched.
- Upgraded wai.common requirement to fix bug with file iteration.