add pixel_shape to WCS converter

[braingram]

Fixes: #474

Schema changes to add pixel_shape to the wcs schema are included in: https://github.com/asdf-format/asdf-wcs-schemas/pull/56

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2b571a9) 87.25% compared to head (f765464) 87.28%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #480 +/- ## ========================================== + Coverage 87.25% 87.28% +0.02% ========================================== Files 22 22 Lines 3814 3821 +7 ========================================== + Hits 3328 3335 +7 Misses 486 486 ```

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[nden]

LGTM. Should a wcs_schemas release be made so this can pick up the change?

[braingram]

Releasing asdf-wcs-schemas once https://github.com/asdf-format/asdf-wcs-schemas/pull/56 sounds good.

The changes here are compatible with the existing 1.1.0 schema (it allows additionalProperties). I don't think the asdf-wcs-schemas release needs to be coordinated with any gwcs release. The conditions for a user could be as follows:

• current asdf-wcs-schema, current gwcs: current behavior (no pixel_shape round-tripping)
• new asdf-wcs-schema (with new wcs manifest), current gwcs: current behavior
• current asdf-wcs-schema, new gwcs: pixel_shape round-tripping (but the written pixel_shape will not be validated as the wcs-1.2.0 schema will not be available)
• new asdf-wcs-schema, new gwcs: pixel_shape round-tripping and validation