Clearly identify properties within parental nodes to which child records can be mapped

[PhilipMusk]

The icdc-model-props.yml file should be updated so as to include an unambiguous identifier of a single property within each parental node used thus far as the key field to which child records must be mapped be mapped during data loading.

These "map to" properties will be identified via a new property definition of "Key:", with its value set to true.