code: public Model deleteVertex(Vertex vertex) {
      edges.removeIf(edge -> vertex.equals(edge.getSourceVertex()) || vertex.equals(edge.getTargetVertex()));
      vertices.remove(vertex);
      return this;
    }
docs: Delete a vertex from the model.
  </p>
  Will remove a vertex from the model. Any edges that has the vertex
  as either source or target, will also be deleted from the model.
  </p>
  
  @param vertex The vertex to be deleted.
  @return The model.