Phase and group delays in Locust

Implement phase and group delays in Locust when there is some time.
![image (15)](https://user-images.githubusercontent.com/7852036/74268402-adb36800-4cbc-11ea-8997-83355829aa9a.png)
![image (16)](https://user-images.githubusercontent.com/7852036/74268403-ae4bfe80-4cbc-11ea-87f0-8e6c3011a25c.png)
 