OperatorTimerAndSample

Operator Timer and Sample

Because there was a dependency in Sample towards the now removed Interval, I had to reimplement Sample in this PR as well.

Note that the new ```OperatorSampleWithTime``` schedules its first sample call before any upstream code is executed. Given the TestScheduler, a sample with 1sec and a timer with 1sec as well won't sample any value. The reason is that in the internal queue of TestScheduler, the sampler's action to take the sample is followed by the action to emit the timer's value.