Make PointSeries CoverageJSON spec compliant

[andreas-grafberger]

Description

Proposes a relatively minimal fix for #122, and changes the PointSeries domainType / TimeSeries encoder/decoder to be CoverageJSON spec compliant.

To keep this focused, I did not experiment with adding back pydantic support, and I only implemented this for TimeSeries, not other encoders/decoders. Those should probably follow in future PRs to avoid any inconsistencies long-term.

Changes:

• Domain axes are now x (longitude), y (latitude), and t. A z axis appears only when there's a real vertical level. A synthetic 0 that was added so far stays in mars:metadata but not in the domain.
• Range axisNames is ["t"] instead of the parameter short name.
• References are split into CRS84 for [x, y], Gregorian TemporalRS for [t], optional VerticalCRS for [z].

Contributor Declaration

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• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.