Allow using a custom separator when merging annotations.

Add an optional argument to OntologyHelper.mergeExtraAnnotations to
allow the caller to specify the separator string to insert between
annotation values, when merging annotations together. The default
separator is a single space character.

This is a code-level feature only. The separator string is _not_
customizable from the command-line, when comment annotations are merged
as part of `--clean-obo merge-comments`.

robot-core/src/main/java/org/obolibrary/robot/OntologyHelper.java

@@ -1829,16 +1829,30 @@ public static void removeExtraAnnotations(OWLOntology ontology, Set<IRI> propert
   }
 
   /**
-   * Merges supernumerary annotations on all entities in the given ontology.
+   * Merges supernumerary annotations on all entities in the given ontology, using a single space
+   * character between each annotation value.
+   *
+   * @param ontology OWLOntology in which to merge supernumerary annotations
+   * @param properties set of annotation property IRIs to merge
+   */
+  public static void mergeExtraAnnotations(OWLOntology ontology, Set<IRI> properties) {
+    mergeExtraAnnotations(ontology, properties, " ");
+  }
+
+  /**
+   * Merges supernumerary annotations on all entities in the given ontology, using a custom
+   * separator between each annotation value.
    *
    * <p>This method iterates over all entities (excluding imported entities) of the given ontology;
    * if an entity is found to have more than one annotation for each of the indicated properties,
    * they are merged into a single annotation.
    *
    * @param ontology OWLOntology in which to merge supernumerary annotations
    * @param properties set of annotation property IRIs to merge
+   * @param separator Separator string to insert between each annotation value (may be {@code null})
    */
-  public static void mergeExtraAnnotations(OWLOntology ontology, Set<IRI> properties) {
+  public static void mergeExtraAnnotations(
+      OWLOntology ontology, Set<IRI> properties, String separator) {
     Set<OWLAnnotationAssertionAxiom> axiomsToRemove = new HashSet<>();
     Set<OWLAnnotationAssertionAxiom> axiomsToAdd = new HashSet<>();
     OWLDataFactory factory = ontology.getOWLOntologyManager().getOWLDataFactory();
@@ -1865,8 +1879,8 @@ public static void mergeExtraAnnotations(OWLOntology ontology, Set<IRI> properti
           Set<OWLAnnotation> axiomAnnotations = new HashSet<>();
           boolean first = true;
           for (OWLAnnotationAssertionAxiom ax : sortAxioms(axioms)) {
-            if (!first) {
-              sb.append(' ');
+            if (!first && separator != null) {
+              sb.append(separator);
             } else {
               first = false;
             }