Wai.annotations.imgstats

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- `label-dist-od` now has `--label-key` option to specify the meta-data key that stores the label
- `label-dist-XY` now sort the labels in alphabetically order before outputting the statistics
- added `area-histogram-is` and `area-histogram-od`

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- moved `to-annotation-overlay-od` into `wai.annotations.imgvis` module

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- added `-s/--scale-to` option to `to-annotation-overlay-od` command to scale all annotations
to a predefined image size.
- overlay output of `to-annotation-overlay-od` is now enforcing PNG format

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- Initial release: label-dist-ic, label-dist-is, label-dist-od, to-annotation-overlay-od