Updated SDF examples

[jennuine]

Added <size> element to ground_plane collisions since it is a required spec of SDFormat in all example and test SDF files.

This also provides collision visualization for the ground plane: #531

[codecov]

Codecov Report

Merging #573 (5e18bb2) into ign-gazebo3 (ddd7fcd) will increase coverage by 0.05%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           ign-gazebo3     #573      +/-   ##
===============================================
+ Coverage        77.66%   77.71%   +0.05%     
===============================================
  Files              208      208              
  Lines            11574    11574              
===============================================
+ Hits              8989     8995       +6     
+ Misses            2585     2579       -6     

Impacted Files	Coverage Δ
src/SimulationRunner.cc	94.53% <0.00%> (+1.12%)	⬆️

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[chapulina]

Nice! Even though DART is ignoring the size and creating an infinite plane, we should still specify it because it is required. Other physics engines may use it.

[azeey]

Isn't the size of the plane only meaningful for visualization? i.e, I thought all physics engines should create an infinite plane. If a plane is not infinite, why is it needed? You can just use a box shape.

[jennuine]

@azeey when trying to view collisions (i.e., visuals) of the ground plane (#531) when the geometry is a //plane, the collision visual could not be viewed because it was missing the //size tag. I did notice in some of the examples used a box shape instead of a plane.

[chapulina]

I thought all physics engines should create an infinite plane. If a plane is not infinite, why is it needed? You can just use a box shape.

I think that's a fair question, but I'm not sure we can make this assumption. A box has 6 sides, while a plane can be interpreted to have 1 or 2 at most. So I can imagine a physics engine that implements a finite 2-sided plane without sides, for example.

That could be something interesting to iterate on the SDF spec.

the collision visual could not be viewed because it was missing the //size tag

That's an interesting use case, because the collision visual may be misleading. Currently, even though the visual will be 100x100, the actual collision created by DART is infinite.