Fixing duration calculation in result events

[danielfeismann]

duration calculation should return duration for the simulation step. At the moment it returns the total duration from start till the end of each step

11:40:00.182 INFO  e.i.s.e.l.RuntimeEventListener$ - ******* Simulation until 2019-01-01T19:00:00Z completed. (duration: 0h : 0m : 27s , memory: 0.1 GB) ******
11:40:01.452 INFO  e.i.s.e.l.RuntimeEventListener$ - ******* Simulation until 2019-01-01T20:00:00Z completed. (duration: 0h : 0m : 28s , memory: 0.2 GB) ******
11:40:02.708 INFO  e.i.s.e.l.RuntimeEventListener$ - ******* Simulation until 2019-01-01T21:00:00Z completed. (duration: 0h : 0m : 29s , memory: 0.1 GB) ******
11:40:03.989 INFO  e.i.s.e.l.RuntimeEventListener$ - ******* Simulation until 2019-01-01T22:00:00Z completed. (duration: 0h : 0m : 30s , memory: 0.1 GB) ******