When composed with multiscales, disallow spatial:transform at top level; require it on all layout items

[kylebarron]

Closes #12

Schema changes

• If multiscales.layout exists, then spatial:transform must not exist at the top level
• If multiscales.layout exists, then spatial:shape must not exist at the top level
• If multiscales.layout exists, then spatial:transform must exist on all items within layout

Change list

• Adds tests for invalid examples, to assert that they are correctly rejected by the schema validator.
  • sentinel2-tci.json is invalid because it doesn't have a spatial:transform on each item in layout.
  • dem-multiresolution.json is invalid because it has a spatial:transform at the top level.

[kylebarron]

We could change this to look for the specific multiscales uuid instead of the fields multiscales and layout, though above it hard-codes multiscales and layout as keys anyways

[geospatial-jeff]

Just make transform and shape required everywhere. It doesn't hurt to be explicit, and it's tiny.

[emmanuelmathot]

Clean, well-scoped change. if/then plus required on layout items reads correctly, and asserting invalid examples are rejected is a nice addition. A few notes:

1. spatial:shape asymmetry. Forbidden at root but not required on layout items either, so it can vanish entirely. Intentional (derivable from the array shape) or oversight? Should mirror spatial:transform if it's normative.
2. README not updated. Existing prose still says these properties "can be added to layout items" / "placed at root", which doesn't reflect the new normative rule.
3. dem-multiresolution-partial.json violates two rules at once (missing on some items and root-level present), so it doesn't independently exercise the "missing on some items" branch. Inline fix below.
4. Will conflict with #24 (removes the 2D/3D oneOf arms); easiest to rebase after that lands.

[emmanuelmathot]

Only spatial:transform is actually required on every layout item, not spatial: properties generally:

Suggested change

	"description": "When composing with multiscales convention, spatial: properties must be added to every layout item. spatial:transform is required on all layout items to unambiguously define the coordinate transformation for each resolution level.",
	"description": "When composing with the multiscales convention, `spatial:transform` is required on every layout item to unambiguously define the coordinate transformation for each resolution level. `spatial:transform` and `spatial:shape` must not appear at the root attributes level in this case.",

[emmanuelmathot]

required below is unconditional inside layout items, so the qualifier is misleading:

Suggested change

	"description": "Affine transform for this resolution level mapping array indices to coordinates. Required on every layout item when using multiscales with this convention.",
	"description": "Affine transform for this resolution level, mapping array indices to coordinates. Required on every layout item.",

[emmanuelmathot]

Drop the root-level spatial:transform so this isolates the "missing on some items" violation:

Suggested change

	"spatial:transform": [
	0.000277778,
	0.0,
	-180.0,
	0.0,
	-0.000277778,
	90.0
	],