use xyz instead of xyzrgb in ClusterPointIndicesDecomposer

[knorth55]

currently, ClusterPointIndicesDecomposer assume that there is XYZRGB field,
so I see this warning when we use point cloud format XYZI again and again

Failed to find match for field 'rgb'.                                                                                                    
Failed to find match for field 'rgb'.                                                                                                    
Failed to find match for field 'rgb'.                                                                                                                                                                                                                                             
Failed to find match for field 'rgb'.                                                                                                                                                                                                                                             
Failed to find match for field 'rgb'.                                                                                                                                                                                                                                             
Failed to find match for field 'rgb'.                                                                                                                                                                                                                                             
Failed to find match for field 'rgb'.                                                                                                                                                                                                                                             
Failed to find match for field 'rgb'.                                                                                                                                                                                                                                             
Failed to find match for field 'rgb'.                                                                                                    
Failed to find match for field 'rgb'.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
Failed to find match for field 'rgb'. 

this PR suppresses the warning.
Because the computation does not use RGB field, so we can use XYZ format for the computation.