Added a parameter to choose the saved file extension (csv,vtk,ply,pcd) a...

I added a parameter to choose the saved file extension (which must be a valid libpointmatcher extension i.e. csv, vtk, ply, pcd). You can either set the parameter with or without the dot (e.g csv or .csv). 

I also added a parameter to change the filename prefix (instead of using the ROS topic name).

If those parameter are not set, they will stay as they were before (i.e. the ROS topic name as prefix and vtk as extension)