Add support for RVL depth compression

[matlabbe]

Once introlab/rtabmap#1409 is merged, rtabmap will be able to decompress RVL depth images internally. So rgbd_sync, rgbd_relay and rgbd_split could now use RVL format when compression is requested.

For other nodes receiving rtabmap_msgs/RGBDImage topics, MsgConversion would need to be updated to support RVL.

Note that when using rtabmap::uncompress with RVL format, it assumes it has a header DEPTHRVL at the beginning of the array of bytes. In image_transport compressed depth plugins, they don't add a signature to the array of bytes (RVL format is set in the encoding field), so we should make sure to add/remove that signature if needed. If we process RVL data directly from ROS, we could use the new rtabmap::RvlCodec class directly to compress/uncompress data without DEPTHRVL signature.

Note that rtabmap library will use RVL by default internally, independently of this PR.