Be strict about adherence to cardinality via collection: true

[ronaldtse]

From:

• The attribute standard_relationship should be a collection relaton/ieee-idams#6

Currently, if an attribute does not define collection: true, lutaml-model accepts all 3 cases:

• 0 occurrences
• 1 occurrence
• N occurrences

However, collection: true means 0..*, and collection: false (default) means 0..1. Right now, the default of 0..1 does not raise any error if there are more than 1 occurrence.

When the XML/JSON does not fit this model definition, it should raise an error.