Introduce a new DeleteUnmatchedReferences option

We have `DeleteUnmatched` options for `Attributes`, `Relationships`, `NestedTopics`, and `Children`. Now, with the introduction of `References`, we need a corresponding `DeleteUnmatchedReferences`, to extend that capability.

Author: JeremyCaney

OnTopic.Data.Transfer/Interchange/ImportOptions.cs

@@ -28,6 +28,7 @@ public class ImportOptions {
     \-------------------------------------------------------------------------------------------------------------------------*/
     private                     bool?                           _deleteUnmatchedAttributes;
     private                     bool?                           _deleteUnmatchedRelationships;
+    private                     bool?                           _deleteUnmatchedReferences;
     private                     bool?                           _deleteUnmatchedChildren;
     private                     bool?                           _deleteUnmatchedNestedTopics;
     private                     bool?                           _overwriteContentType;
@@ -77,6 +78,21 @@ public bool DeleteUnmatchedRelationships {
       set => _deleteUnmatchedRelationships = value;
     }
 
+    /*==========================================================================================================================
+    | DELETE UNMATCHED REFERENCES
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>
+    ///   Delete references in the target database which don't exist in the source <see cref="TopicData"/> graph.
+    /// </summary>
+    /// <remarks>
+    ///   This will delete any existing references that aren't <i>also</i> represented in the source <see cref="TopicData"/>
+    ///   graph.
+    /// </remarks>
+    public bool DeleteUnmatchedReferences {
+      get => _deleteUnmatchedReferences?? Strategy is ImportStrategy.Replace;
+      set => _deleteUnmatchedReferences = value;
+    }
+
     /*==========================================================================================================================
     | DELETE UNMATCHED CHILDREN
     \-------------------------------------------------------------------------------------------------------------------------*/