Fix a number of build warnings

[hellantos]

No description provided.

[destogl]

When you are already on fixing warnings, we just learned this week that -Wconversion flag is not set with -Wall – unfortunately the learning was not easy, and we lost a few hours to debugging issues in the tests caused by implicit casting and narrowing types.

I propose to add this here – just to be sure that all castings are used properly – especially since we are converting a lot between doubles and (u)ints.

check here for more details: ros-controls/ros2_control#1053

[hellantos]

@destogl I've made another issue here #135.