added checks to avoid multiple start calls

Author: endurance21

src/signal.js

@@ -31,6 +31,7 @@ define(function (require) {
    *  @example
    *  <div><code>
    *  let carrier, modulator;
+   *  let hasStarted = false;
    *
    *  function setup() {
    *    let cnv = createCanvas(100, 100);
@@ -57,7 +58,10 @@ define(function (require) {
    *  function canvasPressed() {
    *    userStartAudio();
    *    carrier.amp(1.0);
-   *    carrier.start();
+   *    if(!hasStarted){
+   *      carrier.start();
+   *      hasStarted = true;
+   *    }
    *  }
    *
    *  function mouseReleased() {