Rename ScoreComponent's pattern methods

[ethanroday]

In order to correctly discern between the score's tree and the actual formatted object graph, we must make the following distinction:

• add() will now be the method for the developer-facing score model.

• addChild() will now be the method for the internal composite structure.

• Each class will keep track of a nextLevelDown attribute, so that the implementation of the add() method can be encapsulated in ScoreComponent as:

  add(child) {
      this.nextLevelDown.addChild(child);
  }
  

  Thus, the constructor of ScoreComponent should contain this.nextLevelDown = this as the default, and subclasses can modify if needed. For example, Part should have this.nextLevelDown = this.voiceGroup.