Include <string> to fix compilation error on macOS

[light-tech]

On macOS (maybe generalized to systems using libc++ for the STL), the inclusions of standard headers algorithm and stdexcept do not include string indirectly which leads to compilation error in tricycle_controller/src/steering_limiter.cpp when trying to instantiate an instance const std::string error.

Simply include <string> improves the compatibility with libc++.

[DasRoteSkelett]

lgtm

[codecov-commenter]

Codecov Report

Merging #467 (3b4155d) into master (e7f9962) will decrease coverage by 6.03%.
The diff coverage is 20.21%.

@@            Coverage Diff             @@
##           master     #467      +/-   ##
==========================================
- Coverage   35.78%   29.74%   -6.04%     
==========================================
  Files         189        7     -182     
  Lines       17570      743   -16827     
  Branches    11592      428   -11164     
==========================================
- Hits         6287      221    -6066     
+ Misses        994      162     -832     
+ Partials    10289      360    -9929     

Flag	Coverage Δ
unittests	29.74% <20.21%> (-6.04%)	⬇️

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

Impacted Files	Coverage Δ
...de/diff_drive_controller/diff_drive_controller.hpp	100.00% <ø> (ø)
...ontroller/test/test_load_diff_drive_controller.cpp	12.50% <0.00%> (ø)
diff_drive_controller/src/odometry.cpp	42.16% <11.11%> (ø)
...ive_controller/test/test_diff_drive_controller.cpp	17.62% <12.08%> (ø)
diff_drive_controller/src/speed_limiter.cpp	46.55% <13.33%> (ø)
...troller/include/diff_drive_controller/odometry.hpp	20.00% <20.00%> (ø)
...iff_drive_controller/src/diff_drive_controller.cpp	32.13% <24.11%> (ø)
...ontrollers/src/joint_group_position_controller.cpp
...ory_controller/test/test_trajectory_controller.cpp
...ontrollers/src/joint_group_position_controller.cpp
... and 192 more