new feature: clock sync protocol gtsp (WIP)

see #1538

<del>This PR will add an implementation of the Gradient Clock Synchronization Protocol (GTSP) described in [1]. This PR depends on #1425 .</del>

[1] "Gradient Clock Synchronization in Wireless Sensor Networks", Sommer et.al. 2009

IMPORTANT: I will refactor this code in the next few weeks. I will ping this PR when the code is ready for a
code review. Don't waste your time by reviewing this code now.