Add support for Gazebo9

[naoki-mizuno]

This fixes the problem related to Gazebo's position control, which can be seen
when dropping a position-controlled robot. The issue is fixed by using the
_preserveWorldVelocity argument introduced in Gazebo 9.0.0.

https://bitbucket.org/osrf/gazebo/pull-requests/2814/fix-issue-2111-by-providing-options-to/diff

Related issue/commit in gazebo_ros_pkgs, which introduces a similar fix:

ros-simulation/gazebo_ros_pkgs@3164e4c
ros-simulation/gazebo_ros_pkgs#479
turtlebot/turtlebot_arm#22

Tested on:

• ROS Lunar
• Gazebo 9.0.0

but should work also on Gazebo 7:

Tested on Gazebo7 (where it compiles, but doesn't change anything) and
Gazebo9 (where it compiles and fixes the bug). I've tested it using the
instructions I've put into this repo:

It seems like this PR also fixes the slippery wheels after upgrading to Kinetic. (Maybe related to CIR-KIT-Unit03/cirkit_unit03_simulator#20)

# @MoriKen254 さんのQiitaの記事，いつも読ませていただいてます！

[MoriKen254]

@naoki-mizuno
Thank you very much for your awesome pull request! And I feel so sorry about my late reply.

I just checked your change and it did work without any problem on kinetic! Let me merge your request.

Moreover, I didn't realize the cause of /issues/20 CIR-KIT-Unit03/cirkit_unit03_simulator#20...

According to turtlebot/turtlebot_arm#22, the problem seems to be fixed only on Gazebo 9 doesn't it?
Anyway for now, the system looks to work well! Thank you so much!

ありがとうございます。素人なりに解釈した内容で恐縮です。今後共よろしくお願いします！

[naoki-mizuno]

Thanks for merging!!

the problem seems to be fixed only on Gazebo 9 doesn't it?

Yes, it seems like they didn't backport the fix to Gazebo 7 because it breaks the API (I can't remember where I read it, but someone mentioned it in one of the threads).