A workaround for SLAM-Toolbox, fixed scan data size output mode

[Bob-YsPan]

This pull request provides a workaround for the problem like following:
SteveMacenski/slam_toolbox#63

Add the parameter fixed_scan_size to limit scan_size and angle_increasement to a fixed number and put the scan data to the nearest slots. The final data size is calculated by the number of modes of the lidar scan size and the angle increment value
calculation references the slam_toolbox to meet its needs.

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此pull request为如以下问题的解决方案参考：
SteveMacenski/slam_toolbox#63

添加参数 fixed_scan_size 以将 scan_size 和 angle_increasement 限制为一个固定数字，并将扫描数据放到最近的栏位中。最终的数据大小由激光雷达扫描大小的众数计算得出，角度增量值的计算则参考 slam_toolbox 以满足其需要。