Required Extensions in non root model files #64
Description
https://github.com/3MFConsortium/spec_core/blob/master/3MF%20Core%20Specification.md#34-model
In light of the alternatives extension to the production spec, we should change
Space-delimited list of namespace prefixes, representing the set of extensions that are required for processing the document. Editors and manufacturing devices MUST NOT process the document if they do not support the required extensions.
to
Space-delimited list of namespace prefixes, representing the set of extensions that are required for processing the document. Editors and manufacturing devices MUST NOT process this model-file if they do not support the required extensions.