Wai.annotations

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- Converted formats to use a plug-in system so other libraries can define their own formats.

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- Upgraded wai.common requirement to v0.0.28.
- Separated main settings from library settings.
- Changed ``--include-zero-area`` flag to min/max width/height/area flags.
- Added ``--comments`` option for ROI writer to insert comments at the beginning
of written files.
- Input directories read all contained files ending in ``.report`` when using ``adams`` format.
- Added ``--sample-stride`` option to tfrecords input which sub-samples the mask when generating
the polygon for speed increase.
- Added ``--mask-threshold`` option to tfrecords input which sets the probability to consider
the mask inside the polygon.
- Internal format is now an object rather than a tuple.
- Added ``--size-mode`` flag to ROI format which writes ROI files with x,y,w,h headers rather
than x0,y0,x1,y1 headers.

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- Upgraded wai.common requirement to v0.0.26.

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- VGG/COCO formats can now take a ``--pretty`` flag to pretty-print their JSON annotations.
- Upgraded wai.common requirement to v0.0.25.
- `image_utils.lists_to_polygon` method converts lists of X and Y coordinates into a `planar.Polygon`
- `image_utils.polygon_to_bbox` method returns the x0,y0,x1,y1 coordinates of the bounding box around
the supplied `planar.Polygon` object

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- Conversions now automatically discard annotations with zero area, can be reverted
with ``--include-zero-area`` flag.
- `image_utils.mask_to_polygon` can now work with a view of the mask instead of
the full mask to speed up the polygon detection
- Modified utilities for getting labels/prefixes to be able to specify a default value.
- Upgraded wai.common requirement to v0.0.24.
- Added requirement for wai.json v0.0.4.

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- Added compatibility layer for Tensorflow V1.
- Now ensures Tensorflow is in eager execution mode.