Improved efficiency of pointcloud integration and better parametrization
of sensor frame id.

- Line 310 of OctomapServer.cpp was doing an unnecessary internal copy
(pc_nonground = pc), that can be avoided by using shared pointers.
- Improved assembly of pointcloud from octree by using vector reserve
instead of resize.
- Improved parametrization of sensor frame id. This way the input cloud
can be sent in any frame id (map, sensor, other), and the parameter
sensor_frame_id can be used to specify which is the sensor frame to
compute the origin for the raytracing algorithms (used to integrate the
pointcloud into the octree).