UWSim Updated for ROS Melodic

[dcentelles]

You can either install from the repositories or build UWSim for ROS Melodic. The wiki pages to build, install and run uwsim has also been updated.

http://www.irs.uji.es/uwsim/wiki/index.php?title=Installing_UWSim

After installing/compiling you must run the following commands before the first run to avoid an exception of type 'GeographicLib::GeographicErr':

wget 
https://raw.githubusercontent.com/mavlink/mavros/master/mavros/scripts/install_geographiclib_datasets.sh
chmod u+x install_geographiclib_datasets.sh
sudo ./install_geographiclib_datasets.sh