Ndsampler

Removed
* Drop Python 3.6 and 3.7 support

Changed
* Speed improvements to delayed image loading

Added

* New method `load_target_annots` that just finds overlapping annotations

Added
* `load_sample` now takes an `annot_ids` argument to skip the spatial lookup
and use something precomputed.

* Can now specify `optimize=False` in `load_sample` target dictionary to
prevent delayed image from doing any optimization.

Added
* Experimental support for `rtree` in `isect_indexer`
* Add `verbose_ndsample` option to the target dictionary for debugging

Changed
* Small speedups