add trivial helper to RobotState

[v4hn]

I'm not sure why nobody thought about adding this before...

[codecov]

Codecov Report

Merging #2828 (135e4ac) into master (6012934) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2828      +/-   ##
==========================================
+ Coverage   60.81%   60.81%   +0.01%     
==========================================
  Files         366      366              
  Lines       31717    31717              
==========================================
+ Hits        19284    19287       +3     
+ Misses      12433    12430       -3     

Impacted Files	Coverage Δ
...bot_state/include/moveit/robot_state/robot_state.h	85.04% <ø> (ø)
...ipulation/pick_place/src/manipulation_pipeline.cpp	69.45% <0.00%> (-1.85%)	⬇️
...on/pick_place/src/approach_and_translate_stage.cpp	73.38% <0.00%> (-0.64%)	⬇️
...dl_kinematics_plugin/src/kdl_kinematics_plugin.cpp	77.87% <0.00%> (-0.38%)	⬇️
...nning_scene_monitor/src/planning_scene_monitor.cpp	68.27% <0.00%> (+0.14%)	⬆️
.../ompl_interface/src/detail/constrained_sampler.cpp	59.46% <0.00%> (+16.22%)	⬆️

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