Update gazebo scene viewer tutorial

[iche033]

• Fix compilation against gazebo9. When building the examples, it always includes the Helper.hh header from math4 (gz9 dependency) instead of math6, causing compilation error due to missing math::secNsecToDuration function. The changes make sure that the examples are able to build with either math4 or math6 version.
• moved the Code section to the bottom so that the tutorial layout is consistent with other tutorials

Signed-off-by: Ian Chen ichen@osrfoundation.org

[codecov]

Codecov Report

Merging #147 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #147   +/-   ##
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  Coverage   52.07%   52.07%           
=======================================
  Files         143      143           
  Lines       13105    13105           
=======================================
  Hits         6824     6824           
  Misses       6281     6281           

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[chapulina]

Works for me, I tested against Gazebo 11.

I just have a question and a comment about the image sizes, which is not introduced by this PR.