Update to work with Ubuntu 20.04 and ROS2 Foxy. (#24)

* Update to work with Ubuntu 20.04 and ROS2 Foxy.

* Change CMake to use ament_export_targets instead of ament_export_interfaces. This likely breaks support for ROS2 Eloquent.

* Switch Actions CI from 18.04/Eloquent to 20.04/Foxy.

* Change launch file instances of node-name to name.

* Fix for installing tzdata to accept default option.

* Switch CI to use ROS Docker image instead of Ubuntu Docker image.

* Remove rosdep init since it is already done in ROS image.

* Remove install of packages included in ROS Docker image.

* Revert schedule for CI runs.

* Enable CI on push.

* Remove unused CI environment variable.

* Revert CI distribution variable.

* Fix yaml error for CI. Simplify env variables.

* Remove DEBIAN_FRONTENT.

* Switch scipy use of factorial from misc module to special module.

* More robust way to import factorial.

* Rename all node_* variables in python so that the node_ prefix is removed.

* Switch CI from using Gazebo 9 to 11.

* Updated thruster default axis value

* Launch files now use a topic / param to load the robot description

* Updated thruster_manager to parse an urdf from a string instead of a file to get the thrust axes

* Updated tests to load the robot description from a topic / param

* Updated CI to use both Gazebo 9 and 11

* Fixed syntax error

* Updates about sim time request

* Updated changelog and readme

Co-authored-by: Thomas Denewiler <thomas.denewiler@navy.mil>
Co-authored-by: jpliquid <65894393+jpliquid@users.noreply.github.com>