fixed issue with p5.Score that prevented 'parts' from being passed as argument (#543)

This is related to an issue here: #529

It doesn't look like p5.Score is currently functional because it fails to fill the 'parts' array with the function arguments, due to a conditional that doesn't seem to be necessary.

With this said, is there a way to provide a better error message to provide when p5.Score is missing phrases? Currently when p5.Score is provided no arguments, it returns Cannot read property 'start' of undefined. I imagine just checking if the function call has any arguments would be enough, but not sure if there is a case where that needs to be allowed.

Also, I would be happy to work on converting this to es6 and writing out some tests, if that would be helpful for this class.

Author: aceslowman

src/looper.js

@@ -387,18 +387,16 @@ class Score {
   constructor() {
     // for all of the arguments
     this.parts = [];
-    this.currentPart = 0;
+    this.currentPart = new Array(arguments.length);;
 
     var thisScore = this;
     for (var i in arguments) {
-      if (arguments[i] && this.parts[i]) {
-        this.parts[i] = arguments[i];
-        this.parts[i].nextPart = this.parts[i + 1];
-        this.parts[i].onended = function () {
-          thisScore.resetPart(i);
-          playNextPart(thisScore);
-        };
-      }
+      this.parts[i] = arguments[i];
+      this.parts[i].nextPart = this.parts[i + 1];
+      this.parts[i].onended = function () {
+        thisScore.resetPart(i);
+        playNextPart(thisScore);
+      };    
     }
     this.looping = false;
   }