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 - goal name are correctly printed when no explanation are provided
 - explanation becomes an abstract type

 - create a session if the directory doesn't exists
 - If the directory exists but the xml doesn't behave as if the xml is empty.

Split session in two :
Session : an API for managing session without running provers
Session_scheduler : an API for running provers asynchronously

All the global states have been removed.

A session must be first read, which give a session without task.
Afterward it must be updated to the current state of the files with
some environnement and configuration.

printer and iterator are provided for session.

Session_tools : some useful functions on session.

Smoke detector : not anymore integrated to session. Just add the
      transformation "smoke_detector_top" or "smoke_detector_deep" to
      all the valid proof attempt.

prover_id are not yet removed but all is in place in session for that.