Looks like gicp6d should use PointXYZRGB instead of PointXYZRGBA

Your Environment

• Operating System and version: Ubuntu 18.04
• Compiler: GCC 7
• PCL Version: Latest

Context

I was attempting to use gicp6d for my application and then I realized it only supports point type PointXYZRGBA as input and I am unable to pass in PointXYZRGB point type. To my understanding gicp6d does not consider alpha when doing the alignment and I further look into the code https://github.com/PointCloudLibrary/pcl/blob/cc7fe363c6463a0abc617b1e17e94ab4bd4169ef/registration/include/pcl/registration/gicp6d.h and https://github.com/PointCloudLibrary/pcl/blob/cc7fe363c6463a0abc617b1e17e94ab4bd4169ef/registration/src/gicp6d.cpp there is not a single place that the alpha channel is being used. Before I submit a PR that simply changes PointXYZRGBA to PointXYZRGB in gicp6d.h, @taketwo and @SergioRAgostinho is there anything that I misunderstood that makes PointXYZRGBA required?

EDIT: I made the changes locally and it seems to be working just fine.

Start by comparing registration/include/pcl/registration/gicp6d.h with registration/src/gicp6d.cpp, focusing on the PointXYZRGBA input type and whether alpha is referenced. Confirm that PointXYZRGB can be used without affecting alignment, then verify the relevant gicp6d behavior and ensure the requested point type is supported consistently.