make Vertex serializable

src/main/java/GraphMLFileWriter.java

@@ -35,7 +35,7 @@ public class GraphMLFileWriter {
             GraphMLExporter<Vertex, SimpleWeightedGraph<Vertex, DefaultWeightedEdge>> exporter = new GraphMLExporter<>(Vertex::getVertexLabel);
             //set to export weights
             exporter.setExportEdgeWeights(true);
-            //set type, sequnce, and occupancy attributes for each vertex
+            //set type, sequence, and occupancy attributes for each vertex
             exporter.setVertexAttributeProvider( v -> {
                 Map<String, Attribute> attributes = new HashMap<>();
                 attributes.put("type", DefaultAttribute.createAttribute(v.getType()));

src/main/java/Vertex.java

@@ -1,6 +1,6 @@
+import java.io.Serializable;
 
-
-public class Vertex {
+public class Vertex implements Serializable {
     private  SequenceType type;
     private  Integer vertexLabel;
     private  Integer sequence;