find the admissible bounds for a model and its database
simul_bounds=bounds_db4model(model,dbmod)
* model = a model tlist
* dbmod = a database tsmat
* simul_bounds = a (n x 3) string matrix, with:
- the first column gathers the first possible starting date
- the second column gathers the last possible starting date
- the third column gathers the last possible ending date