Wai-annotations

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- Fixed default filename for ROI format ("-rois.csv" rather than "-roi.csv").
- Added the ability to specify a prefix and suffix for reading/writing ROI files.
- Logging changes.
- Moved utilities for each format into their own sub-package.
- Added reader options to read input files from list files.

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- Separated command-line option parsing from io/converter classes so that imports
unique to specific formats are not required unless using that format.
- Refactoring of package structure.
- Added logging of when files are being written.
- Conversion to COCO format now report polygon mask area, rather than just bounding-
box area.
- Added support for non-standard keywords to the ROI format.

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- Added the ability to specify inputs by glob syntax, and also specify negative
images with no annotations.
- Added utility functions for images (image_to_numpyarray, remove_alpha_channel)
and for masks (mask_to_polygon, polygon_to_minrect, polygon_to_lists) in module wai.annotations.image_utils
- Added --extensions option to control the search order for associated images.
- Added --verbosity flag to control logging verbosity.
- Added polygon mask support to the ROI format.

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- Modified requirements to include Cython as this is required in later versions
of pycocotools. Moved pycocotools to end of dependency list to ensure all
requirements are met before installation.

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- Changed --coerce option to --force.
- Tensorflow is no longer a dependency in setup.py as can work with
tensorflow or tensorflow-gpu.
- TFRecords format can now handle polygon-mask annotations.

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- Can now work with images that contain no annotations.