selects the naive pairing algorithm when shapes are not on disk

examples/use_api/create_session.ml

src/ide/gmain.ml

src/session/session.ml

src/session/session.mli

src/session/session_scheduler.ml

src/session/session_scheduler.mli

src/session/termcode.ml

src/session/termcode.mli

src/tools/why3replay.ml

src/why3session/why3session_csv.ml

src/why3session/why3session_html.ml

src/why3session/why3session_info.ml

src/why3session/why3session_latex.ml

src/why3session/why3session_lib.ml