Update solver.py: better handling of "verbose" option

Better handling of "verbose" parameter in solver.init():

• if the parameter evaluates to False (or 0), verbose.Not is used ;
• if the parameter is True (or 1), verbose.Yes is used (default) ;
• if the parameter is 2, verbose.Yes is used.

I replaced self.parameters["verbose"] by self.verbose, for two reasons:

• there is only one option ("verbose") for now, so keeping a dict with only one value is useless, and it is easier afterwards to use self.verbose than self.parameters["verbose"] ;
• the dict already exists in init, it is kwargs.