New OOT module gr-latency

Hi everyone,

I'd like to share a new module `gr-latency` that I created and use[0].

Sometimes it would be great to be able to measure latency in a
flowgraph. With this module this should be easier. I have questions in
mind like:

* Which blocks delay processing?
* How long does it take a packet to propagate through the DSP chain?

This module is inspired by [1]
"latency: Tag samples or messages with timestamps and measure how long
it takes to propagate them through the flowgraph."

Cheers
Johannes

[0] https://github.com/ant-uni-bremen/gr-latency
[1] https://github.com/bastibl/gr-sched#metrics