Functions that allow simple plotting to rviz from Python.

Documentation is sparse, but to get started, try the following:

1. Start roscore
2. Start rviz
3. In rviz, import the example configuration by opening rviz, clicking File-->Open, then selecting the path/to/rviz_pyplot/etc/rviz_pyplot.rviz file from the rviz_pyplot source directory. This does the following:
   1. It sets "Global Options-->Fixed Frame" to rviz_pyplot
   2. It creates a new MarkerArray display listening to the topic /rviz_pyplot/marker_array, and
   3. It creates a new PointCloud2 display listening to the topic /rviz_pyplot/points.
4. Run the example scripts in the rviz_pyplot/examples folder. Important: You must run these scripts from inside a python interpreter (use ipython and the magic command "run"). If you try to run the scripts from the command line, the script exits before the message is sent and nothing will appear in rviz.
5. Now read the scripts to get an idea of what magic is happening there.