incremental mode vs multi-goals: currently, work is done twice

[iguerNL]

Once https://github.com/OCamlPro/alt-ergo/pull/371 is merged, Alt-Ergo will support SMT2 incremental mode (push/pop).

But the way things are done in

• psmt2_to_alt_ergo (translates check-sat and check-sat-assuming commands to goal)
• main_text (all context reassumed for successive goals in multi-goal mode)

is not optimal.

The main loop in main_text should be improved.

[iguerNL]

When the handling in main_text is simplfied, a particular attention should be given for check and cut commands