Delayed-image

Added
* Can now work with different `warp_affine` backends.

Fixed

* Fixed issue where an xarray could come back without specified channels

Fixed
* LICENSE issue

Changed
* Minor optimizations
* Added `__slots__` to all node classes
* Warping now uses `origin_convention='corner'` when warping. This is a
breaking change, but the result should be minor for any window size with more
than a few dozen pixels, and it the surrounding code was previously
incorrectly assuming the convention was corner.

Fixed

* Fixed unexpected results caused by incorrect assumptions that the origin convention was corner.

Changed:
* Added the `lazy` keyword argument to `crop` and `warp`, which will attempt to avoid overhead of creating a new layer if the operation is a no-op.
* Minor speed improvements
* Can now specify `num_overviews` to DelayedLoad to bypass metadata preparation.

Removed

* Drop Python 3.6 and 3.7 support.

Fixed

* Fixed issue where optimizing an implicitly cropped warp and overview would drop the implicit crop.