Create a real-time clock topic for the Python nodes

[Georacer]

As far as I know, Python nodes do not support Wall* timing routines. This inhibits code development in Python in some cases.

Could this be solved by a dedicated clock topic for use by Python nodes? Something like a C++ node witch triggers off of the WallClock and publishes to a topic at a predefined rate. Python nodes could subscribe/trigger off of this topic to run at a realtime clock. If this topic is of Time type, then possibly time operations could be used.