Store full PC2 in Rtabmap Database

[Nosille]

While I understand that there are significant computational and storage efficiencies associated with the built-in pcl datatypes (e.g., pcl::PointCloudpcl::PointXYZRGBNormal), I find these datatypes very limiting. I am a ROS user, and I frequently find myself wishing RTABMAP would simply store the full original PointCloud2 object in the database. As a result, I have been working on a modified version of RTABMAP that adds three new database entries, pc2_info, pc2_fields, and pc2_data. These entries are simply a breakdown of a PCLPointCloud2 objects into blobs that can be stored in a sql database. I am close to having a usable product and would love the official RTABMAP team's advice and feedback moving forward. My hope is that this approach will provide a lot of flexibility for data storage. It many cases, I believe it will provide a very straight forward path for capturing user generated data.

https://github.com/Nosille/rtabmap/tree/AddPc2ToDatabase
https://github.com/Nosille/rtabmap_ros/tree/AddPc2ToDatabase

I am currently creating the scan and scan_info blobs consistent with the main branch and simultaneously populating the pc2_info, pc2_fields, and pc2_data blobs with the full original PCLPointCloud2 object. While this means that basic pointcloud information is stored twice right now, I believe the efficiency of pcl::PointCloud objects for common operations justifies keeping the scan/scan_info blobs, and I wasn't ready to couple the scan and pc2 database entries in a way that would allow the pc2 data to exist without the x, y, z fields. Hopefully, it is something that can be improved in the future.