Add Support for Custom Point Structure with XYZ-RGB-A Data

Need support for a custom point structure in PointCloud Library (PCL) to include:

• XYZ (position data)
• RGB (color data)
• A (alpha/transparency channel)

Example

struct PointXYZRGBA {  
    float x, y, z;  
    uint8_t r, g, b, a;  

    EIGEN_MAKE_ALIGNED_OPERATOR_NEW  
};  

POINT_CLOUD_REGISTER_POINT_STRUCT(PointXYZRGBA,  
                                  (float, x, x)  
                                  (float, y, y)  
                                  (float, z, z)  
                                  (uint8_t, r, r)  
                                  (uint8_t, g, g)  
                                  (uint8_t, b, b)  
                                  (uint8_t, a, a))