[Task] Validate against multiple aspects in one file

[chris-volk]

The rule "An aspect model file may contain at most one model element definition for samm:Aspect." from https://github.com/eclipse-esmf/esmf-semantic-aspect-meta-model/blob/main/documentation/decisions/0007-model-resolution.md (Aspect Model File may not contain more than one aspect) is fundamental for further artefact generation, usage in Aspect Model Editor etc.
Hence it should be considered during validation, validation should fail with reference to this rule if not kept.