spatial:shape on multiscale parent groups #355

Reported in #349 (G3).

The five multiscale parent groups (/measurements/reflectance, /quality/atmosphere, /quality/probability, /conditions/mask/l1c_classification, /conditions/mask/l2a_classification) have spatial:transform of the finest level, but no spatial:shape. Without the shape, a reader cannot tell what the transform applies to.

generic_multiscales gives the shape to grid_spatial_attrs (s2_optimization/s2_multiscale.py:465-468). grid_spatial_attrs uses it to compute the bbox, but it does not write spatial:shape (s2_multiscale.py:490-511).

Proposal

Write spatial:shape in grid_spatial_attrs. This also fixes the other callers.

Also discussed in zarr-conventions/spatial#31 and zarr-conventions/spatial#32 (one source for each spatial: property).

Tests

Regenerate the S2 snapshots. Add a test: each group with spatial:transform also has spatial:shape.