Array API: port the healpy/healpix functions #403
Description
Is Your Feature Request Related to a Problem? Please Describe
As identified in #383. Unsure if healpy/healpix plan to support the Array API.
Probably want to have a generic namespace that uses, e.g. healpy if CPU is required, and otherwise uses a suitable GPU backend if requested. The logic is likely to be similar to the S2FFT package.
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Describe Alternatives You've Considered
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Additional Context
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