Added fs2000d.mod in the testsuite (test of Marco's optimization routines).

- no longer use the 'noclearall' option when calling Dynare
- test the simulation path for the various stack_solve_algo options
- simplify code

Preprocessor: in steady_state_model block, allow MATLAB functions which return several arguments (closes #37)

fixed bug that would force node sharing when two or more expectation operators had the same expression but different information sets

Testsuite: reorganize for bock and bytecode, using a script for systematically testing options combinations

- fail when user tries to initialize a variable at a period which is not in the model, and make the error message explicit
- correctly handle exogenous variables in stochastic mode, which are also substituted with an aux var
- added test files

options (closes #98)