Add a RealTimeWorldNode for displaying simulations at real-time speeds

[mxgrey]

I recently needed to make a real-time simulation display on top of dartsim, so I decided to clean it up and add it into the main repo since it's a frequently requested feature (e.g. #1204).

It's a simple wrapper on top of the dart::gui::osg::WorldNode class. Users can opt for either the old WorldNode or the new RealTimeWorldNode depending on how they want to control the speed of the simulation.

I also added a line ImGui::CreateContext(); because ImGui was crashing without that.

I know #1215 has already been opened, so I can retarget this at a 6.8 release (or we can wait until 6.7.1 since this won't impact API or ABI) instead if that would be preferred.

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• Document new methods and classes
• Format new code files using clang-format

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• Set version target by selecting a milestone on the right side
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[mxgrey]

I also looked over the osg examples and replaced their use of WorldNode with RealTimeWorldNode for any examples that perform simulation. I think the new playback speed looks much better.

[jslee02]

Looks great and thanks for the fix of the crashing with ImGui! Let's ship 6.7 with this new feature.

I left a few nitpick comments. One another suggest I have is formatting the new file using clang-format. You could do that by adding these line:

dart_format_add(
  RealTimeWorldNode.hpp
  RealTimeWorldNode.cpp
)

at the end of dart/gui/osg/CMakeLists.txt, and run make format with clang-format-3.8 installed. If you're on bionic, then clang-format-3.8 isn't available. In that case, I can do it myself.

[jslee02]

LGTM!

[jslee02]

Hm, the last commits from me are in incorrect order due to the wrong push dates. This is because the date setting of my virtual machine was set to 2 days late for some reason. 😞