[Data] Add tif file extension to ImageDatasource

[bveeramani]

Why are these changes needed?

tif is a valid image file extension, but read_images doesn't recognize it because it's not included in the ImageDatasource._FILE_EXTENSION list.

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[scottjlee]

Assuming that the underlying library used to read the image files supports .tif as well.

[bveeramani]

Assuming that the underlying library used to read the image files supports .tif as well.

Yeah, PIL works with .tif.