Fix min occurs for choice content types

[tefra]

📒 Description

The dtd mapper is not setting the min_occurs restriction correctly for choice elements.

Resolves #703

🔗 What I've Done

Write a description of the steps taken to resolve the issue

💬 Comments

This is still not perfect for cases like this, where elements appear in both choices, the attribute merger will set these fields as list which technically it's not really correct.

<!ELEMENT PaymentRequestDetail (
(PaymentDocumentID , PaymentSequenceNo? , PaymentDates? , 
(
    FinancialInstitutionDetail|
    (OriginatingFinancialInstitution , ReceivingFISpecificID)|
    (OriginatingFISpecificID , (ReceivingFinancialInstitution|ReceivingFISpecificID))|
    CardInfo
) 

🛫 Checklist

• Updated docs
• Added unit-tests
• Sample tests pass
• W3C tests pass

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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