Bugfix in InteractiveMarkerViewer

[aditya-vk]

If the viewer has an underlying world, skeleton markers corresponding to skeletons in the world that have been removed continued to be visualized. When the world removes the skeleton, the SkeletonPtr is non-nullptr and therefore the control did not reach the line that erases the marker from the map. This PR reorganizes the code for correction.

Tested via herb3_demos scripts for addition, deletion and updates to skeletons.

@brianhou: As a side: I wonder if we should re-introduce the WorldInteractiveMarkerViewer, not in its previous state but just to reimplement the updateSkeletonMarkers() function to have fewer if conditions.

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[brianhou]

Suggested change

	for (const FrameMarkerPtr& marker : mFrameMarkers)
	for (const auto& marker : mFrameMarkers)

[egordon]

@brianhou @aditya-vk This introduces a warning (if statement ambiguity) that breaks all downstream Travis CI calls (that treat warnings as errors):

/root/workspace/src/aikido/src/rviz/InteractiveMarkerViewer.cpp: In member function 'void aikido::rviz::InteractiveMarkerViewer::updateSkeletonMarkers()':
/root/workspace/src/aikido/src/rviz/InteractiveMarkerViewer.cpp:81:30: error: suggest parentheses around '&&' within '||' [-Werror=parentheses]
     if (!it->first || mWorld && !mWorld->hasSkeleton(it->first))
                       ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors