Add -Wconversion flag to protect future developments (backport #667)

[mergify]

Same reasons as ros-controls/ros2_control#1053

explicitly catching errors of time being zero in forward_command_controller see
I think there might be an ABI breaking in JTC with one argument changed from int to size_t. I did not know what to do there, and went for ABI breaking but I could also make it a static_cast if preferred. see

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This is an automatic backport of pull request #667 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 23f0def has failed:

On branch mergify/bp/iron/pr-667
Your branch is up to date with 'origin/iron'.

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Unmerged paths:
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	both modified:   diff_drive_controller/src/diff_drive_controller.cpp
	both modified:   joint_trajectory_controller/src/joint_trajectory_controller.cpp
	both modified:   tricycle_controller/src/tricycle_controller.cpp

no changes added to commit (use "git add" and/or "git commit -a")

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[christophfroehlich]

Oops, this was already included in iron.