Do not override empty URDF link collision geometry

[Dale-Koenig]

Partially addressing #130. Add a warning so that more drastic action can be taken when this issue is revisited in the future, and to ensure that the user is aware that the collision geometry has been altered.

Fixes #130 and fixes #1907. Changes to no longer override collision geometry.

[codecov-io]

Codecov Report

Merging #1952 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1952   +/-   ##
=======================================
  Coverage   49.82%   49.82%           
=======================================
  Files         313      314    +1     
  Lines       24714    24715    +1     
=======================================
+ Hits        12313    12314    +1     
  Misses      12401    12401           

Impacted Files	Coverage Δ
moveit_core/robot_model/src/robot_model.cpp	76.64% <100.00%> (-0.08%)	⬇️
...ove_group_interface/src/wrap_python_move_group.cpp	43.03% <0.00%> (ø)
...ls/include/moveit/py_bindings_tools/gil_releaser.h	100.00% <0.00%> (ø)

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[v4hn]

Thank you for the patch. The maintainer meeting actually agreed to a more drastic patch in the master branch, but no one acted on it yet.

Could you change your patch to keep the warning, but not add the collision geometry?

The reasoning was as follows:

• If we want to change the standard, we should do this in the master branch directly. This is what the branch is for.
• If we add the warning and anyone really relies on the old behavior, they will just have to copy their geometry to the collision geometry too
• If people want the new behavior, they will have to ignore the warning for some transition period.

Also, please add a helpful entry to the MIGRATION.md?

[v4hn]

I would actually propose to set this warning to a different logger name, so that affected users can set this logger to ERROR if they want to ignore the specific warning.
How about LOGNAME+".empty_collision_geometry"?

Suggested change

	ROS_WARN_STREAM_NAMED(LOGNAME, "Link " << urdf_link->name << ": "
	"has visual geometry but no collision geometry. "
	"Collision geometry will be left empty. "
	"Fix your URDF file by explicitly specifying collision geometry.");
	ROS_WARN_STREAM_NAMED(LOGNAME+".empty_collision_geometry", "Link " << urdf_link->name << ": "
	"has visual geometry but no collision geometry. "
	"Collision geometry will be left empty. "
	"If you require collision checking for the link, explicitly set the collision geometry in the URDF."
	"Otherwise, consider disabling this logger name via rosconsole.");

[rhaschke]

I added some minor cleanup. Looks good. Thanks!