Support partial reads of compressed and uncompressed blocks

[braingram]

For applications that are only interested in small portions of large arrays it would be beneficial to have some API for reading portions of a block.

This may relate to: #1871

For example, let's say we have an ASDF file with a large array "data" (4k x 4k) and we're only interested in a small known portion of that array (a 30x30 pixel window at 42, 42). Some questions are:

• What is a helpful API for supporting this operation? (perhaps https://docs.astropy.org/en/stable/io/fits/api/images.html#astropy.io.fits.ImageHDU.section can provide some inspiration)
• How to handle compressed blocks (do all or any of the current compression algorithms support this with lz4 being of particular interest)
• Are there particular block usage patterns that will be difficult to support (streams, views, etc)?