Ndsampler

Changed
* Using the new kwcoco delayed operation backend when sampling

Added
* Environment variable `NDSAMPLER_DISABLE_OPTIONAL_WARNINGS` can disable warnings

* tr can now take parameters interpolation and antialias which are now passed
to kwcoco delayed finalize.


Changed
* Setting nodata to "float" or "auto" makes nan the default pad value.
* `new_sample_grid` can now take extra kwargs that are passed to the video or
image sample grid function. Added a `use_annots` flag that can disable the use
of annotations in constructing the grid.
* `new_sample_grid` now returns a list "targets" that contains all targets and
a list of positive and negative indexes that indicate which of them are
positive or negative if `use_annots` is specified. The old "positives" and
"negatives" items in the return dict still exist but are deprecated and will be
removed.

Added
* Add `dtype` argument to `load_sample`, which will control the type of data returned.
* Add `nodata` argument to `load_sample`, which will handle nodata values for integer images.


Changed
* using the experimental loader is now the faster method (given kwcoco >
0.2.13) and has been set as the default, and the other method has been removed.

Fixed

* Bug where the video sample grid did not respect "video-space"