more fixes for nested chunks
more fixes for nested chunks

- fixed method to get data associated with chunk ID
- isComplete now works for root type chunk

shamrt/jsPsych,jodeleeuw/jsPsych,jspsych/jsPsych,wolfgangwalther/jsPsych,andyh616/jsPsych,adrianoesch/jsPsych,shamrt/jsPsych,erikasf/jsPsych,vergenzt/jsPsych,jspsych/jsPsych,msrresearch/jsPsych,adrianoesch/jsPsych,wolfgangwalther/jsPsych,erikasf/jsPsych,jspsych/jsPsych,andyh616/jsPsych,vergenzt/jsPsych,jspsych/jsPsych,Scienthusiast/jsPsych,Scienthusiast/jsPsych