Not all structures (especially private ones) have an ostream operator because I
did not need them this time, not for any technical reason

(one useful commit...)

This reverts commit 3e994c89.

This reverts commit 2edb055d.

This reverts commit 1e8102d9.

More specifically take into account the new headers of
abstract-robot-dynamics.

Reinforce policy to not externalize internal types.
This will help contributors by allowing internal
changes without impacting users.

Please DO NOT put types in the API which are not necessary
to interact with the user. This should stay in the private
area.

This reverts commit c8afc3f3.