Issue #14: Convert Engine class to Singletion pattern

Signed-off-by: Lucas Fernandes de Oliveira <lfo14@inf.ufpr.br>

src/core/engine.spec.ts

@@ -27,7 +27,7 @@ import { View } from "./view";
 import { engineScenario } from "../../test/scenario";
 
 describe("engine class", () => {
-    const engine = new Engine();
+    const engine = Engine.getInstance();
 
     const met = engineScenario.metrics;
     const dim = engineScenario.dimensions;

src/core/engine.ts

@@ -25,11 +25,21 @@ import { Query } from "../common/query";
 import { RelationType } from "../common/types";
 
 export class Engine {
+    private static instance: Engine;
+
     private views: View[] = [];
     private metrics: Metric[] = [];
     private dimensions: Dimension[] = [];
 
-    constructor () { }
+    private constructor () { }
+
+    public static getInstance(): Engine {
+        if (this.instance == null) {
+            this.instance = new Engine();
+        }
+
+        return this.instance;
+    }
 
     public getViews() {
         return this.views;