Adopt fully array-api-spec compliant code

[Oisin-M]

Is your feature request related to a problem? Please describe.

There are still a number of places where we have to convert code to be array-api-compliant. This issue is to track progress on this

• use of xp.fmax and xp.fmin (fix/avoid_xp_fmax #62)
• use of xp.arctan2 (chore: use atan2 instead of arctan2 as per array-api-spec #63)
• use of xp.deg2rad (chore: replace deg2rad with array-api-spec version #64, feat: add deg2rad and rad2deg methods to patched namespaces earthkit-utils#27)
• use of eku array backends in testing instead of using array-api-compliant approach (Use backends only for testing earthkit-utils#16)
• vertical module using numpy only

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