global:
-------
- put some documentation in .mli files
- try some more heuristics to extract models for simplexes over ints.
- add the ability to do some encoding (with a flag): (eg. cubes-test
for LIA)
- add the ability to backtrack by directly relaxing bounds. This would
allow to not rely on the functional style to backtrack, and to keep
the pivots and valuation that have been computed so far

- add the ability to apply substitutions resulting from pivots
lazily. This is very important for scalability

- optimization: early detection of unsat if one of the variables in
  fixme cannot be fixed

assertBounds.ml:
----------------

- assert_var and assert_poly should ignore bounds when Pb is UNSAT

- assert_var and assert_poly should ignore bounds that are implied by
  those of the simplex

- add a mode for assert_var and assert_poly to ignore previously
  asserted bounds ? (i.e. mode force_bounds) useful for assertUI.ml ?

- deduce bounds from the simplex ?
