Array API: units/Quantity handling with non-numpy arrays

[mwcraig]

Problem

astropy.units.Quantity is numpy-backed, so any path that attaches units to image data pulls the data out of the user's array library.

Evidence:

• The workaround in flat_correct, which strips and re-applies units manually to avoid Quantity:

  ccdproc/ccdproc/core.py

  Line 991 in 9d25eee

  # TODO: Fix this when astropy supports array namespaces

• CuPy issues in test_combiner #911 failures of the form TypeError: unsupported operand type(s) for <<: 'ndarray' and 'IrreducibleUnit' in scaling-related combiner tests — a CuPy array being shifted into a Quantity.

• The related TODO about CCDData arithmetic being numpy-based:

  ccdproc/ccdproc/core.py

  Line 1016 in 9d25eee

  # TODO: Workaround for the fact that CCDData currently uses numpy

Task

Define one strategy for units with non-numpy data — the current wrapper approach (_CCDDataWrapperForArrayAPI keeps units beside the data rather than wrapping it) seems right, but it needs to cover the combiner scaling paths too. Audit every place a Quantity is constructed from image data and route it through the same mechanism. Track astropy's own array-API plans for units in #940.

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Found during a review of the array API implementation from #885; follow-up to #909 / #911.