Use inline less operator for PointXYZIFR

The less operator is highly used when sorting rays, thus the overhead can be reduced by inlining it according to the Cpp-Core-Guidelines (https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c86-make--symmetric-with-respect-to-operand-types-and-noexcept).

While we do see some unexpected variations in our measurements (especially for the total runtime of each module) which we are still investigating, generally this set of patches improves the performance for us. For example, for our test system (Intel Core i7-6700HQ @ 2.60GHz × 8, 40 GB RAM, Ubuntu 18.04, ROS 2 Dashing, Autoware.auto (Hash: dba2158e)), the following performance improvements could be measured:

ray_ground_classifier: ~x1.7
velodyne_cloud: ~x1.7
(as a combination of the branches: walbroel.add_point, walbroel.iterative_quick_sort, walbroel.less_operator and walbroel.fast_atan2).