Kwcoco

Added

* Added "domain net" to `kwcoco grab`.
* Added "spacenet7" to `kwcoco grab`.


Fixed
* Fixed issue in `add_annotation` where if failed to add keypoints, and segmentations.

Added

* `CocoDataset.index.trackid_to_aids` is a new index that maps a track-id to
all annotations assigned to that track.

* `kwcoco grab` CLI for downloading and converting standard datasets

* `CocoDataset.delayed_load` now has a limited ability to specify a list of
channels to load, and if it is returned in image or video space.

Changed
* `kwcoco.add_annotation` can now accept and automatically convert kwimage data
structures for segmentation and keypoints.

Fixed
* Channels in a base image can now be None.
* `validate` now prints which field caused the issue

Changes
* Updated README
* Code cleanup
* More docs

Fixes
* Fixed error with empty union
* Fixed error in `delayed_load`

Added

* Experimental `delayed_load` method for an image

Fixed
* Fixed `cats_per_img` in kwcoco stats.
* Fixed `grab_camvid`.
* Fixed legacy `img_root` key not being respected.

Changed
* Made bezier dependency optional
* stats now returns video stats

Added

* Added `channel_spec.py` as a helper and documentation
* Auxiliary images now have `warp_aux_to_img` property
* Base images now have `warp_img_to_vid` property

Changed

* Removed `base_to_aux` transform in favor of inverse `warp_aux_to_img`


Fixes
* Fixed error in kwcoco conform documentation
* The conform operation now warns if the segmentation cannot be converted to a polygon