Extend `RegionDetection/Set` and implement NMS

[russelldj]

Most of this PR is making changes/extensions to the RegionDetections and RegionDetectionsSet classes to support streamlined NMS.

• Add an explicit merge method to RegionDetectionSet to handle the logic of merging multiple regions together or returning a list of per-region detections.
• Rename get_detections to get_data_frame
• Add a method to get a copy of a RegionDetections with a subset of detections.
• Make the instantiation of a RegionDetections more flexible so inputs can be provided in geospatial coordinates but also have a transform to pixels associated.

Also I currently have un-used functionality to get the bounds of the predicted region in different CRS for both the RegionDetections and RegionDetectionSet.

[russelldj]

I also implemented NMS, which was fairly straightforward. One major limitation is this library only supports rectangular bounding boxes. Currently, this is defined as the minimum bounding rectangle for each polygon (.bounds). In the future we might want to support real polygon NMS. Also, there's an option to include a multi-class variation in LSNMS, the library we use. I didn't implement it, but we could consider it in the future.