added native sensor_msgs::PointCloud2 support without conversion

[spuetz]

Since the conversion from PointCloud2 to PointCloud and back for assembling the clouds is bad, I added a native sensor_msgs::PointCloud2 support without conversion. I also added an organized mode, to assemble PointClouds2 to an organized PointCloud2.

[vrabaud]

pcl is a huge dependency for that simple function. Use tf2_sensor_msgs instead: https://github.com/ros/geometry_experimental/blob/indigo-devel/tf2_sensor_msgs/include/tf2_sensor_msgs/tf2_sensor_msgs.h#L59

[vrabaud]

overall very useful, thank you. It will probably take use a few days to get through the code. I believe some code should be part of the base class (as private lightweight functions). E.g., all that code is duplicated for PointCloud and PointCloud2:

if (field->offset != this->scan_hist_[start_index].fields[chan_ind].offset){

Please have only one function for those (like a function that return analyses two different PointFields)

[vrabaud]

@spuetz , any comments on that ?

[spuetz]

@vrabaud, I added the method hasEqualScanHeader to check the headers. as recommended by you. Additionally, I fixed the down-sampling for the organized mode.

[spuetz]

@vrabaud, would be nice, if we can complete this pull request and merge it. thx