Fix Typescript error in force_directed_graph.ts (#74)

Fix Typescript error in force_directed_graph.ts

src/force_directed_graph.ts

@@ -8,6 +8,7 @@ import * as d3 from 'd3';
 import {AVisInstance, IVisInstance, assignVis, IVisInstanceOptions} from 'phovea_core/src/vis';
 import {mixin} from 'phovea_core/src';
 import GraphProxy from 'phovea_core/src/graph/GraphProxy';
+import { GraphNode } from 'phovea_core/src/graph/graph';
 
 export interface IForceDirectedGraphOptions extends IVisInstanceOptions {
   /**
@@ -17,6 +18,13 @@ export interface IForceDirectedGraphOptions extends IVisInstanceOptions {
   colors?: boolean;
 }
 
+/**
+ * Extends the D3 Node with a Phovea GraphNode property
+ */
+interface IMixedNode extends d3.layout.force.Node {
+  v: GraphNode;
+}
+
 export class ForceDirectedGraphVis extends AVisInstance implements IVisInstance {
   private readonly $node: d3.Selection<any>;
 
@@ -107,7 +115,7 @@ export class ForceDirectedGraphVis extends AVisInstance implements IVisInstance
     const colors = d3.scale.category10().range().slice();
 
     this.data.impl().then((graph) => {
-      const nodes = graph.nodes.map((n) => ({v: n}));
+      const nodes: IMixedNode[] = graph.nodes.map((n) => ({v: n}));
       const lookup = d3.map(nodes, (d) => String(d.v.id));
       const edges = graph.edges.map((n) => ({
         v: n,