Improvement of processing speed about ultralytics_ros

[Alpaca-zip]

I've implemented parallel processing using OpenMP in the feature/omp_parallel branch. Testing with the KITTI dataset, the average processing time in syncCallback() improved from 17.5 ms to 14.8 ms, marking an average improvement of 15.5%.

My configuration is as follows:

• WSL Ubuntu-20.04 ROS Noetic
• Intel(R) Core(TM) i7-10750H CPU @ 2.60GHz 2.59 GHz
• 16.0GB RAM
• NVIDIA GeForce RTX 2060

@h-wata, if you have some time, could you please test it in your environment as well? If you notice a significant improvement and find this change beneficial, I'll consider merging it into the noetic-devel branch.
Thank you for your cooperation.

[h-wata]

Thank you for your implementation.
When I set voxel_leaf_size:=0.01, it takes 2.0 seconds to calculate projectCloud() in the noetic-devel. By the way, in the feature/omp_parallel branch, it takes between 0.6 to 0.7 seconds.
Furthermore, the CPU usage is three times higher than in the noetic-devel when three objects are shown.

However, this setting is quite aggressive. With the default setting of 0.1, both branches should have no problem regarding computation time.