Add ability to crop raster source extent

[AdeelH]

Overview

Allows different portions of the same scene image to be used in different dataset splits.

• Adds an extent_crop field to RasterSourceConfig that is passed on to the RasterSources.
  

  raster-vision/rastervision_core/rastervision/core/data/raster_source/raster_source_config.py

  Lines 23 to 33 in 1d278f8

  	extent_crop: CropOffsets = Field(
  	None,
  	description='Relative offsets '
  	'(skip_top, skip_left, skip_bottom, skip_right) for cropping '
  	'the extent of the raster source. Useful for splitting a scene into '
  	'different dataset splits. E.g. if you want to use the top 80% of the '
  	'image for training and the bottom 20% for validation you can pass '
  	'extent_crop=CropOffsets(skip_bottom=0.20) to the raster source in '
  	'the training scene and extent_crop=CropOffsets(skip_top=0.80) to the '
  	'raster source in the validation scene. Defaults to None i.e. no '
  	'cropping.')

• Updates RaterioSource and MultiRasterSource to make use of extent_crop.
• Some minor improvements to Box().
• Adds unit tests.

Checklist

• Updated docs/changelog.rst
• Added needs-backport label if PR is bug fix that applies to previous minor release
• Ran scripts/format_code and committed any changes
• Documentation updated if needed
• PR has a name that won't get you publicly shamed for vagueness

Testing Instructions

• How to test this PR
  • See new unit tests

[lewfish]

Thanks for the tests! It's a little confusing that the coordinate system is flipped for top vs. bottom and left vs. right. Eg. top=0.2 and bottom=0.8 refer to the same point. Being a little more explicit in the docs would be helpful.

[AdeelH]

Thanks for the tests! It's a little confusing that the coordinate system is flipped for top vs. bottom and left vs. right. Eg. top=0.2 and bottom=0.8 refer to the same point. Being a little more explicit in the docs would be helpful.

(top=0.2, left=0.0, bottom=0.0, right=0.0) means skip the top 20% of the image and use the bottom 80%.

(top=0.0, left=0.0, bottom=0.8, right=0.0) means use the top 20% and skip the bottom 80% of the image.

[AdeelH]

I have updated it to use a NamedTuple and elaborated the usage a bit more in the description.

[AdeelH]

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