Allow specifying POINT_TYPE as cmake option

[CyrillePierre]

Currently, in the CMakeLists.txt, the POINT_TYPE is defined using

#=======================================
# Custom Point Type (XYZI,XYZIRT, XYZIF, XYZIRTF)
#=======================================
set(POINT_TYPE XYZI)

As explained in the documentation, to change its value, the user have to edit this file.
I believe that users should not have to edit any project file just to choose a particular configuration. This would requires them to create a fork of the project to maintain their changes.

Instead, I propose to let the user select the POINT_TYPE in a similar way than other options like ENABLE_DIFOP_PARSE or ENABLE_IMU_DATA_PARSE. It is still possible to edit the CMakeLists.txt, but this PR allows to specify the POINT_TYPE from the command line. For example, in ROS2 with colcon:

colcon build --packages-select rslidar_sdk --cmake-args -DPOINT_TYPE=XYZIRTF

To summarize, I changed the following things:

• I have added a condition to check if POINT_TYPE is already defined in the CMakeLists.txt
• I have updated the documentation to explain the different way to set its value

However, I do not speak Chinese, so I do not have edited this version of the documentation.

[jfrascon]

I have a repo that integrates PRs from this official repo: https://github.com/jfrascon/rslidar_sdk
Also I have this repo with Docker images for usual sensors used in robotics (that might be useful for you): https://github.com/jfrascon/ros2_sensor_docker_images