Design of the reference construct

[larsolofsson]

See current reference construct in issue #302.
When working with the reference construct, I have found that papiNet need to discuss some improvements.

When an item is referenced, e.g. a DeliveryNote, then the identifier of the item can be an id (UUID) or a number assigned by a party.
Currently the name of the reference type is ending with "Number", e.g. DeliveryNoteNumber.
The proposal is to remove "Number" from the the name of the reference type, e.g. to be just DeliveryNote.

Property Id (UUID) will not be used in use cases in version 5.0.0.
It can be added later, if needed in the future
The proposal is to remove property Id: (UUID) from the reference construct.

Name change of property assignedByIdentifierBySupplier?
Would assigningPartyIdentifierBySupplier or assignedByPartyIdentifierBySupplier be a better name?
The proposal is to change name .assignedByIdentifierBySupplier to assigningPartyIdentifierBySupplier

The proposal is to make the reference constract more compact and more flat by applying above proposals and replacing one object with 2 properties.

 references:
 type: array
 minItems: 1
 items:
 type: object
 unevaluatedProperties: false
 required:
 - type
 - assignedBy
 - assigningPartyIdentifierBySupplier
 - number
 properties:
 type:
 description: The type of the referenced item.
 type: string
 enum:
 - DeliveryNote
 - DeliveryInstruction
 - LoadingInstruction
 - Shipment
 assignedBy:
 description: The business role of the assigning party.
 type: string
 enum:
 - Forwarder
 - Supplier
 - WarehouseOperator
 assigningPartyIdentifierBySupplier:
 type: string
 minLength: 1
 description: The identifier of the assigning party assigned by the _Supplier_.
 number:
 type: string
 minLength: 1
 description: The identifier of the referenced item given by the assigning party.
 dateTime:
 type: string
 format: date-time
 description: |
 The dateTime when the referenced item was created or updated. 
 It MUST be local or UTC date-time expressed in ISO 8601
 sequenceNumber:
 type: integer
 minimum: 0
 description: A sequenceNumber in the referenced item.
 lineItemNumber:
 type: integer
 minimum: 0
 description: A lineItemNumber in the referenced item.

Current reference construct

 references:
 type: array
 minItems: 1
 items:
 type: object
 unevaluatedProperties: false
 required:
 - type
 - assignedBy
 anyOf:
 - required:
 - id
 - required:
 - identifier
 properties:
 type:
 type: string
 enum:
 - DeliveryNoteNumber
 - DeliveryInstructionNumber
 - LoadingInstructionNumber
 - ShipmentNumber
 assignedBy:
 description: The business role of the assigning party.
 type: string
 enum:
 - Forwarder
 - Supplier
 - WarehouseOperator
 id:
 type: string
 format: uuid
 identifier:
 type: object
 unevaluatedProperties: false
 required:
 - value
 - assignedByIdentifierBySupplier
 properties:
 value:
 type: string
 minLength: 1
 assignedByIdentifierBySupplier:
 type: string
 minLength: 1
 description: The identifier of the assigning party assigned by the _Supplier_.
 dateTime:
 type: string
 format: date-time
 description: It MUST be local or UTC date-time expressed in ISO 8601
 sequenceNumber:
 type: integer
 minimum: 0
 lineItemNumber:
 type: integer
 minimum: 0