Fix current action goal resetting.
Move goal handle status setting to controller.

- Basic state constraint validation remains in segment header file.

- Goal handle status management now lives in controller.

- Document constraint validation classes/functions.
Better printing of time values.
Move controller-specific code to its namespace.

...and out of the trajectory_interface namespace.
Trivial doc addition.
Trivial doc fix.
Rename file for more consistent naming.
Deprecate multi_dof_segment.

- Its functionality has been introduced in the QuinticSplineSegment rework that
  made it inherently multi-dof.
Add goal handle value checking to init tests.
Rename constraints -> tolerances in code.
Unit-test trajectory state tolerance checking.
Expose Scalar type as segment typedef.
Scalar type fully templated accross the board.

To make this happen, and good for other reasons as well: separate State in
QuinticSplineSegment to a separate file.
Add missing \endcode in doc.
init method is now part of the Segment public API.
Make starting() method realtime-safe.