[JTC] Renaming variables, reordering trajectory checks and fixing open-loop mode by adding last commanded time.

[destogl]

This PR increseas readability of the JTC by renaming trajectory varialbes to be semantically correct and self explainable.

Also it reoders checks of the trajectories done in the callback. They are grouped based on the things they are checking, i.e., 1. data available, 2. time checks; 3. trajectory point checks.

Additionally, it adds "last_commanded_time_" for the open loop mode to be more correct when setting point before trajectory.

[mergify[bot]]

This pull request is in conflict. Could you fix it @destogl?

[bmagyar]

Recently merged the wraparound feature which caused some conflicts.

Also, this may be interesting for @fmauch

[christophfroehlich]

:eyes: @destogl test_no_jump_when_state_tracking_error_not_updated fails now, I guess due to the last_commanded_time_ change.

[bmagyar]

Please adjust / extend the tests