Livox support

[fratopa]

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Hi! I am very impressed by you wonderful SLAM project. I am trying to see if it can run using the pointclouds coming from a mems lidar, in particular the Mid-360. Reading your original paper it seems like it should be quite generalizable but looking through your codebase I see that you always check for pcl::PointCloud<pcl::PointXYZRGB> while most lidars only output XYZ values. And in fact when testing it using my data I get the errors:

[pcl::KdTreeFLANN::setInputCloud] Cannot create a KDTree with an empty input cloud!
[pcl::KdTreeFLANN::setInputCloud] Cannot create a KDTree with an empty input cloud!
Failed to find match for field 'rgb'.
Failed to find match for field 'rgb'.
Failed to find match for field 'rgb'.
[pcl::KdTreeFLANN::setInputCloud] Cannot create a KDTree with an empty input cloud!
[pcl::KdTreeFLANN::setInputCloud] Cannot create a KDTree with an empty input cloud!
Failed to find match for field 'rgb'.
Failed to find match for field 'rgb'.
Failed to find match for field 'rgb'.

Am I missing something?