Produce one or more mapping strings for the given property's value.
This is a virtual method, to be overridden by subclasses of Persona who have linkable properties.
Each of their linkable properties should be handled by their implementation of this function, examining the current value of the property
and calling callback with one or more mapping strings for the property's value. Each of these mapping strings will be added
to the IndividualAggregator's link map, related to the Individual instance which
contains this Persona.
0.6.0