changed map and waypoints saving approach

Given that we now use the strands_systems installation and we should not be writing on /opt/strands, I changed the map and waypoint saving. The user now specifies where to save the files using  arguments when calling roslaunch. The reading of maps and waypoints follows the same approach.

The README  was completely updated as it was for an older version.

@cdondrup you did the last big changes here, wanna see what you think about it?