add function to set use_sim_time parameter

[andermi]

call this->use_sim_time() or self.use_sim_time() to set node clock to use /clock from gz sim via ros_gz_bridge

[andermi]

@rhenthorn after using self.use_sim_time() in your class __init__,

seconds, nanoseconds = self.get_clock().now().seconds_nanoseconds()

will give you the sim time to use when writing log files

[hamilton8415]

This brings me into a part of this code that I'm not very familiar with. But, looks straightforward and the best test will be use in the log files. I'm doing a lot of runs with the simulator and faster than real time for the logs will be great.

[andermi]

I'm going to go ahead and merge since it won't affect anything currently being used.