Add support for passive joints - joints with only state interfaces

[destogl]

The PR if functional but the code is not tested against all use cases, especially those with mimic joints.
I don't think there are any issues, but this changes has to be extensivelly tested before merging.

[christophfroehlich]

I tested this successfully with the basic examples in this repo, but what is the goal of this change @destogl? Passive joints with state interfaces only worked already before?

[destogl]

I tested this successfully with the basic examples in this repo, but what is the goal of this change @destogl? Passive joints with state interfaces only worked already before?

This PR separates commanding and state joints. The issue I was having was crashing of plugin when joint didn't have command interfaces, but only state interfaces. But you said this was working… Have to test this again then. I was using Gazebo Sim later for the project, so I am not sure anymore what the exact details behind it.

[christophfroehlich]

See #172 : This is the way I used it since foxy up to now.. Never had problems with that.