Allows an experiment to be continuously repeating at n seconds interval

Finished 2085

- Call Repeat(second, func) with second > 0 will spawn a goroutine to run task continuously with n second interval. It returns a *RepeatItem, which allows stopping the repeating timer e.g. RepeatItem.Stop()

- Repeat(0, func) will run the task immediately without using gorountine, and nil will be returned