Avoid publishing joint messages with duplicate stamps

[MarkHedleyJones]

This pull request prevents the ypspur ros node from publishing joint state messages with duplicate timestamps.

[at-wat]

[86] PASSED on noetic

All tests passed

build/test_results/ypspur_ros/roslint-ypspur_ros.xml: 1 tests
build/test_results/ypspur_ros/rostest-test_test_joint_trajectory.xml: 1 tests
build/test_results/ypspur_ros/rosunit-test_joint_trajectory.xml: 2 tests
Summary: 4 tests, 0 errors, 0 failures, 0 skipped

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (32a36c1) 23.91% compared to head (23d20f8) 23.95%.

Files	Patch %	Lines
src/ypspur_ros.cpp	25.00%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
+ Coverage   23.91%   23.95%   +0.04%     
==========================================
  Files           3        3              
  Lines         690      693       +3     
==========================================
+ Hits          165      166       +1     
- Misses        525      527       +2     

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[at-wat]

LGTM