Extend dunder methods and code simplification for annotations (#251)

* Add z_index to geojson properties when importing and exporting
* Fix annotation crop for Polygons in read_region and test_annotations
* Created new AnnotatedGeometry, adhere to shapely dunder and functions
* Add helper function to utils and Enum for Darwin/ASAP annotation types
* Add darwin_annotation_type to color look up and move Darwin util function to utils
* Bugfix for WsiAnnotations.filter function

dlup/_image.py

@@ -26,8 +26,8 @@
 
 from dlup._exceptions import UnsupportedSlideError
 from dlup._region import BoundaryMode, RegionView
-from dlup.backends.common import AbstractSlideBackend
 from dlup._types import GenericFloatArray, GenericIntArray, GenericNumber, GenericNumberArray, PathLike
+from dlup.backends.common import AbstractSlideBackend
 from dlup.utils.backends import ImageBackend
 from dlup.utils.image import check_if_mpp_is_valid