Added reading of TIFF child images

[radarhere]

Helps #6482 by adding reading of TIFF child images - a series of offsets to child IFDs.

There are two test files here.

• One was created using tifffile, with https://forum.image.sc/t/tifffile-ome-tiff-generation-is-taking-too-much-ram/41865/9 as a basis. For that image we can just seek to the child IFD and load the TIFF image.
• The other test image uses the outdated JPEGInterchangeFormat. Because "No new TIFF writer code should ever attempt to use this tag", I didn't think I was going to find a way to create a small file in this format, and instead simply appended a JPEG to a TIFF file with Pillow, setting the offset to the JPEG image accordingly.