Fix admittance controller interface read/write logic (backport #1232)

[mergify]

Using ROS rolling and gazebo Harmonic, I've found two bugs while trying to control a robot using both velocity and position control.

1. In write_state_to_hardware(), state_commanded.positions are mistakenly being written to both velocity and acceleration command interfaces.
2. Both in write_state_to_hardware() and read_state_from_hardware(), checking whether the controller has pos/vel/acc interfaces is done using if-else clauses. This causes the block to exit upon first true condition, meaning, after confirming position interface exists, velocity and acceleration interfaces never get written to or read from

These fixes are verified in my use case, but seem obvious enough that no tests need to be written to verify these claims.

Nonetheless, let me know if you'd like some tests before merging.

Cheers!

---

This is an automatic backport of pull request #1232 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 87.31%. Comparing base (1cdaacb) to head (1d69718).

Files	Patch %	Lines
...dmittance_controller/src/admittance_controller.cpp	0.00%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             iron    #1235      +/-   ##
==========================================
+ Coverage   87.29%   87.31%   +0.01%     
==========================================
  Files          92       92              
  Lines        8408     8405       -3     
  Branches      701      702       +1     
==========================================
- Hits         7340     7339       -1     
+ Misses        812      808       -4     
- Partials      256      258       +2     

Flag	Coverage Δ
unittests	87.31% <0.00%> (+0.01%)	⬆️

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

Files	Coverage Δ
...ntroller/test/test_trajectory_controller_utils.hpp	84.00% <ø> (ø)
...dmittance_controller/src/admittance_controller.cpp	74.39% <0.00%> (ø)

... and 2 files with indirect coverage changes