When Why3 is run on a file where some theories have been suppressed, it
will delete the corresponding theories from the session file.  We now
add an option keep_unmatched_theories to Session.update_session, which
keeps all theories. In this commit, this option is always disabled.

This is useful for SPARK, which sometimes only generates part of the
Why3 file for efficiency reasons, but doesn't want the session file to
be damaged because of that.

* session.ml
(import_theory)
(import_goal)
(import_proof_attempt)
(import_transf): new functions to copy a session tree from an old
  session file
(merge_file): keep old theories when keep_unmatched_theories is true
* session_scheduler.ml
(update_session): pass keep_unmatched_theories
* why3session_lib.ml
(read_update_session): pass keep_unmatched_theories