SeaDataNet vocabulary context pull

[pzaborowski]

some of the SeaDtaNet aligned examples like this:
https://github.com/ogcincubator/iliad-apis-features/blob/master/_sources/zarr_attrs_sdn/examples/.zattrs-LATITUDE

is referring to the NERC vocabulary in their context files:
https://github.com/ogcincubator/iliad-apis-features/blob/master/_sources/zarr_attrs_sdn/context.jsonld
-> "http://vocab.nerc.ac.uk/collection/P31/current/?_profile=nvs&_mediatype=application/ld+json"
which includes e.g. grid_mapping definition, but the toolkit still generate the mapping to the default fallback default-context

grid_mapping:
type: string
x-jsonld-id: https://w3id.org/iliad/oim/default-context/grid_mapping

while it shall find the http://vocab.nerc.ac.uk/collection/P31/current/grid_mapping/
from https://raw.githubusercontent.com/ogcincubator/iliad-apis-features/refs/heads/master/_sources/zarr_attrs_sdn/P31.json
or http://vocab.nerc.ac.uk/collection/P31/current/?_profile=nvs&_mediatype=application/ld+json

• I've put copy of the NERC files on this repo to eliminate potential problems with files access though the web (one of contexts has 80MB).