display difference between reference clock and each tf node, which was supported until groovy

frame_view until groovy shows reference clock and how much each node in tf tree differ from that[1], which is very convenient when we use ros over multiple machines and find out which machine needs to be ntpdate-ed. but from hydro, that information is not displayed[2].

This patch shows that information, but not work for simulation environment which we need to read '/clock' and if I understand correctly the tf2 is designed not to depend on ROS communication framework. But still this helps when we working on real robot.

[1]
![screenshot from 2014-12-06 13 40 10](https://cloud.githubusercontent.com/assets/493276/5326217/5cdc9bd8-7d4e-11e4-840e-e3601aa98a4d.png)

[2]
![screenshot from 2014-12-06 13 42 16](https://cloud.githubusercontent.com/assets/493276/5326216/4ca0313a-7d4e-11e4-893c-0d8fe4a29428.png)


Ok, could you give me any hint to get current time in tf2 library?

--
◉ Kei Okada


On Wed, Dec 17, 2014 at 3:12 AM, Tully Foote <notifications@github.com>
wrote:
>
> In tf2/src/buffer_core.cpp
> <https://github.com/ros/geometry_experimental/pull/73#discussion-diff-21918479>
> :
>
> > @@ -170,6 +170,7 @@ BufferCore::BufferCore(ros::Duration cache_time)
> >    frameIDs_["NO_PARENT"] = 0;
> >    frames_.push_back(TimeCacheInterfacePtr());
> >    frameIDs_reverse.push_back("NO_PARENT");
> > +  ros::Time::init();
>
> This module is specifically designed to be ROS independant as much as
> possible. Use of ros::Time was specifically designed out because it does
> not work in the case of a python wrapper. (ros::Time only works if roscpp
> is running. in pytf implementations ros::Time::now() does not work in
> simulated time) We need to find a way to implement this without adding that
> dependency back.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/ros/geometry_experimental/pull/73/files#r21918479>.
>