TOTG: add zero parameterization for single-waypoint trajectories

[v4hn]

Also add some overdue helpers to RobotState while I was at it.
I'm happy to discuss naming if someone disagrees with my terminology.

The actual TOTG issue came up in:
#2052 (comment)

[codecov-io]

Codecov Report

Merging #2054 into master will decrease coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2054      +/-   ##
==========================================
- Coverage   54.61%   54.53%   -0.09%     
==========================================
  Files         328      328              
  Lines       25597    25624      +27     
==========================================
- Hits        13980    13974       -6     
- Misses      11617    11650      +33     

Impacted Files	Coverage Δ
...bot_state/include/moveit/robot_state/robot_state.h	91.02% <ø> (ø)
moveit_core/robot_state/src/robot_state.cpp	44.40% <0.00%> (-1.22%)	⬇️
...cessing/src/time_optimal_trajectory_generation.cpp	75.83% <0.00%> (-0.37%)	⬇️
...e/src/parameterization/model_based_state_space.cpp	70.34% <0.00%> (-2.76%)	⬇️
moveit_core/robot_model/src/joint_model_group.cpp	59.39% <0.00%> (-2.43%)	⬇️
...nning_scene_monitor/src/planning_scene_monitor.cpp	70.95% <0.00%> (+0.44%)	⬆️
...meterization/work_space/pose_model_state_space.cpp	82.99% <0.00%> (+0.68%)	⬆️
...anning_scene_monitor/src/current_state_monitor.cpp	52.82% <0.00%> (+1.53%)	⬆️

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