Add methods to remove entities from Node

[jacobperron]

When an entity is disposed, make sure to also remove it from the Node.
This resolves an issue where invalid entities may be used by other classes or users.
For example, a disposed Subscription being accessed by the executor.

Fixes #105

[esteve]

@jacobperron there seems to be an error in flake8 itself, which causes the tests for rosidl_generator_java to fail. I recall this same issue a while ago was caused by pytest using a non-public API from flake8, either updating ament_flake8 or pinning flake8 to an older version fixed the issue.

2020-06-08T23:30:54.4839662Z 3: Test command: /usr/bin/python3 "-u" "/opt/ros/dashing/share/ament_cmake_test/cmake/run_test.py" "/home/runner/work/ros2_java/ros2_java/ros_ws/build/rosidl_generator_java/test_results/rosidl_generator_java/flake8.xunit.xml" "--package-name" "rosidl_generator_java" "--output-file" "/home/runner/work/ros2_java/ros2_java/ros_ws/build/rosidl_generator_java/ament_flake8/flake8.txt" "--command" "/opt/ros/dashing/bin/ament_flake8" "--xunit-file" "/home/runner/work/ros2_java/ros2_java/ros_ws/build/rosidl_generator_java/test_results/rosidl_generator_java/flake8.xunit.xml"
2020-06-08T23:30:54.4839876Z 3: Test timeout computed to be: 60
2020-06-08T23:30:54.4840274Z 3: -- run_test.py: invoking following command in '/home/runner/work/ros2_java/ros2_java/ros_ws/src/ros2_java/rosidl_generator_java':
2020-06-08T23:30:54.4840738Z 3:  - /opt/ros/dashing/bin/ament_flake8 --xunit-file /home/runner/work/ros2_java/ros2_java/ros_ws/build/rosidl_generator_java/test_results/rosidl_generator_java/flake8.xunit.xml
2020-06-08T23:30:54.4840888Z 3: Traceback (most recent call last):
2020-06-08T23:30:54.4841008Z 3:   File "/opt/ros/dashing/bin/ament_flake8", line 11, in <module>
2020-06-08T23:30:54.4841357Z 3:     load_entry_point('ament-flake8==0.7.11', 'console_scripts', 'ament_flake8')()
2020-06-08T23:30:54.4841699Z 3:   File "/opt/ros/dashing/lib/python3.6/site-packages/ament_flake8/main.py", line 75, in main
2020-06-08T23:30:54.4841827Z 3:     max_line_length=args.linelength)
2020-06-08T23:30:54.4843475Z 3:   File "/opt/ros/dashing/lib/python3.6/site-packages/ament_flake8/main.py", line 163, in generate_flake8_report
2020-06-08T23:30:54.4844163Z 3:     style = get_flake8_style_guide(flake8_argv)
2020-06-08T23:30:54.4845687Z 3:   File "/opt/ros/dashing/lib/python3.6/site-packages/ament_flake8/main.py", line 131, in get_flake8_style_guide
2020-06-08T23:30:54.4846031Z 3:     application.parse_preliminary_options_and_args([])
2020-06-08T23:30:54.4848128Z 3: AttributeError: 'Application' object has no attribute 'parse_preliminary_options_and_args'
2020-06-08T23:30:54.4848509Z 3: -- run_test.py: return code 1
2020-06-08T23:30:54.4848963Z 3: -- run_test.py: generate result file '/home/runner/work/ros2_java/ros2_java/ros_ws/build/rosidl_generator_java/test_results/rosidl_generator_java/flake8.xunit.xml' with failed test
2020-06-08T23:30:54.4849427Z 3: -- run_test.py: verify result file '/home/runner/work/ros2_java/ros2_java/ros_ws/build/rosidl_generator_java/test_results/rosidl_generator_java/flake8.xunit.xml'
2020-06-08T23:30:54.4849571Z 3/6 Test #3: flake8 ..........................................................***Failed    0.55 sec

Edit: never mind, I just saw your message in #109 (comment)

[jacobperron]

#111 disables the flake8 check for rosidl_generator_java. We can re-enable it for Eloquent/Foxy CI jobs (when we add them), and for Dashing once it is fixed upstream.

[clalancette]

One pretty minor thing; otherwise looks good to me.

[jacobperron]

I've rebased to fix CI 🤞

[jacobperron]

@esteve @clalancette PTAL

[esteve]

@jacobperron thanks!