WIP: decompress into array

[braingram]

Description

The goal here is to read in bytes from a file into an existing array. Something like the following:

import asdf
import numpy as np

arr = np.ones(1000, dtype='i4')

af = asdf.AsdfFile({"arr": arr})
af.set_array_compression(arr, 'bzp2')
af.write_to("arr.asdf")

dst = np.zeros(2000, dtype='i4')
with asdf.open("arr.asdf") as af:
    breakpoint()
    af._blocks.blocks[0].load(out=dst[1000:].view(dtype='uint8'))

assert np.all(dst[:1000] == 0)
assert np.all(dst[1000:] == 1)

This works with the current commit but there are a bunch of loose ends.

Checklist:

• pre-commit checks ran successfully

• tests ran successfully

• for a public change, added a towncrier news fragment

  changes/<PR#>.<changetype>.rst

  • changes/<PR#>.feature.rst: new feature
  • changes/<PR#>.bugfix.rst: bug fix
  • changes/<PR#>.doc.rst: documentation change
  • changes/<PR#>.removal.rst: deprecation or removal of public API
  • changes/<PR#>.general.rst: infrastructure or miscellaneous change

• for a public change, updated documentation

• for any new features, unit tests were added