Enable JTC for hardware having offset from state measurements

[destogl]

This PR replaces the first part of #161.

The goal is short:

• support "open-loop" control when hardware state is not exactly following the robot commands, i.e., there is systematic offset in the state measurements
• add approach to change controller without "jumps" from the command to state values:
  • when started HW interface has to set command_interfaces to N aN
  • controller checks if command interfaces are NaN --> then is a controller started for the first time, so read states
  • if command interface have a value --> so a controller was running already (supports switching between controllers)

[codecov-commenter]

Codecov Report

Merging #189 (c5109c9) into master (e7f9962) will decrease coverage by 0.78%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
- Coverage   35.78%   34.99%   -0.79%     
==========================================
  Files         189       27     -162     
  Lines       17570     2709   -14861     
  Branches    11592     1803    -9789     
==========================================
- Hits         6287      948    -5339     
+ Misses        994      152     -842     
+ Partials    10289     1609    -8680     

Flag	Coverage Δ
unittests	34.99% <ø> (-0.79%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ler/test/test_load_joint_trajectory_controller.cpp
...s/test/test_load_joint_group_effort_controller.cpp
...ntroller/test/test_trajectory_controller_utils.hpp
...nt_state_controller/src/joint_state_controller.cpp
...ontrollers/src/joint_group_position_controller.cpp
...ory_controller/test/test_trajectory_controller.cpp
..._broadcaster/test/test_joint_state_broadcaster.cpp
...test/test_load_joint_group_velocity_controller.cpp
...ectory_controller/test/test_trajectory_actions.cpp
...ntroller/test/test_trajectory_controller_utils.hpp
... and 206 more

[bmagyar]

Mostly nitpick, otherwise looks good to me!

[v-lopez]

LGTM, just minor comment that can be ignored.