Describe a structured way to handle a strictContainmentAggregationStructure

[skinkie]

First appearing recently in #588

Going through NeTEx the strictContainmentAggregationStructure is handled differently and certainly not consistently.

I think we want to assure:

1. there is at minimum a single element in this containment structure, otherwise it must be invalid.
2. I think this can be assured with a choice minOccurs=1 (the default) also on its sub elements.

[skinkie]

Have an in depth discussion with the group if we should restrict (certain) containment structures to a single type.

https://github.com/NeTEx-CEN/NeTEx/pull/588/files#r1425169764

For example:
valididityConditions: [AvailabilityConditionRef] XOR [AvailabilityCondition]
calls: [DatedCall] XOR [Call]

[nick-knowles]

There are some further check that could be made for consistence and correctness

1. check that strictContainmentAggregationStructureis used where it should be (and not onetoManyRelationshipStructure)

2. VersionedChild should generally be contained within a strictContainmentAggregationStructure

[ue71603]

@skinkie do we do this in 2.0 or 2.1? May it affect "the interface".. Probably yes...