Avoid push_back within getAttachedBodyObjects

[AndyZe]

This is a pretty small optimization but I don't like to see push_back() called within a function that should be performant. This should be faster if there are multiple attached collision bodies.

[codecov]

Codecov Report

Merging #2732 (8f33cac) into master (e414de8) will increase coverage by 0.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2732      +/-   ##
==========================================
+ Coverage   60.42%   60.72%   +0.30%     
==========================================
  Files         402      402              
  Lines       29633    29635       +2     
==========================================
+ Hits        17904    17994      +90     
+ Misses      11729    11641      -88     

Impacted Files	Coverage Δ
.../collision_detection_fcl/src/collision_env_fcl.cpp	89.75% <100.00%> (+0.11%)	⬆️
...meterization/work_space/pose_model_state_space.cpp	82.06% <0.00%> (-0.64%)	⬇️
...e/collision_detection_fcl/src/collision_common.cpp	78.18% <0.00%> (+0.26%)	⬆️
...raint_samplers/src/default_constraint_samplers.cpp	84.22% <0.00%> (+0.33%)	⬆️
...dl_kinematics_plugin/src/kdl_kinematics_plugin.cpp	81.41% <0.00%> (+0.42%)	⬆️
...on/pick_place/src/approach_and_translate_stage.cpp	73.95% <0.00%> (+0.71%)	⬆️
...ipulation/pick_place/src/manipulation_pipeline.cpp	73.79% <0.00%> (+1.95%)	⬆️
.../move_group_interface/src/move_group_interface.cpp	47.32% <0.00%> (+3.39%)	⬆️
...s/include/moveit/py_bindings_tools/serialize_msg.h	95.00% <0.00%> (+5.00%)	⬆️
...ove_group_interface/src/wrap_python_move_group.cpp	37.82% <0.00%> (+10.95%)	⬆️
... and 1 more

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[v4hn]

might be that fewer elements are needed than reserved (if createCollisionGeometry returns nullptr), but yes, reserving is much better. 👍