Converts a database into a tsmat
tsm=db2tsmat(db,arg1,...,argn)
* db = a string indicating the path and the name of the database
* argi = optional arguments:
- 'testna' if the users wants to test the na values in the data (default: not tested)
- 'dropna' if the users wants to remove the periods when one at least of the series contains na values (default: kept)
- 'keepall' if the users wants to keep real vectors in the database (their length must then be equal to the length of the time period of the ts)
* tsm = a tlist of type tsmat