Update row_step when using copyPointCloud on PCLPointCloud2

Updates the row_step field on the output PCLPointCloud2 to reflect
changes in point cloud size. 

According to the [PointCloud2 documentation](http://www.ros.org/doc/api/sensor_msgs/html/msg/PointCloud2.html), `data` should have size equal to `(row_step*height)`. Clearly, this is almost always false if `cloud_out.row_step` is simply copied from `cloud_in`. This patch updates `cloud_out.row_step` according to the number of points selected by `indices`.