changing docker file & removing travis

fix clearing logic for near-max_obj_height instances

Re-enable dwb_plugin tests

- execute gzclient and gzserver individually
- use an argument named headless to execute gzclient

Multiple controller plugins in server for contextual control 

* Attempt to filter out generated message files from code coverage.

* Reduce the number of tests run in Travis to just a single test.

* Filter specific msg packages instead of ros_idl... directories.

* Fixing a typo in the nav_2d_msgs package name.

added ros2-devel as dependency

renamed hpp and cpp files to current bt v3 naming conventions

* added Turtlebot 3 Waffle Gazebo model with depth camera plugin

*changed the robot's shape. used box for the body and cylinder for the wheels 

* removed visuals from urdf. 

*removed meshes from the Gazebo model.

Clean shutdown of system tests

robot_spawner -> nav2_gazebo_spawner

* Switch to use action interface instead of topic.

* Shutdown nav-stack once test has been completed.

* Have lifecycle manager reset managed noded in the correct order

* Add comment about race condition on action server during shutdown

* Listener should be reset before the buffer.

* Making bt nodes separate libraries.

* Using params file to specify which plugins to use.

* Add plugin names to multi-robot params files.

Multiple planner plugins server for contextual planning

* Update params for obstacle layer using poincloud

* Enabled Voxel Layer

* voxel param file

* Update param file

* Remove duplicate param yaml & fix voxel reset

* Short voxel layer documentation

* observation sources in individual layer namespaces

* Added missed param change

* Updated documentation

* Documentation for multiple sensors

* Add subscribe count checker before publishing costmap.

* Fix linter errors

* Address PR feedback.

* Add count subscriber to costmap-update publisher and costmap-grid publisher

* cleanup all globally hardcoded remaps for multirobots

* fixing map path

* fix pep8 indent

* linter happy now?

* Fix linter errors

* reincluding /scan on scan

* multirobot launch with unique configurations

* multirobot tests using both configuration files

* make flake8 happy

* flake8 try2

* flakey9

* flake10

* attempt flake exception

* flake11

* flake12

* noqa on lin

* changing launch configuration in loop

* nav2_controller short documentation

* Added nav2_core short documentation

* minor correction indicating action server

* cleanup