add pixel_shape to WCS converter

[braingram]

Fixes: #474

Schema changes to add pixel_shape to the wcs schema are included in:
asdf-format/asdf-wcs-schemas#56

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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 asdf-format/asdf-wcs-schemas#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