Saves a model description (its parameters) to a file.
Usage: save(file_name [,
attribute])
where file_name is the name of the file into which the description
will be saved; and attribute is an optional parameter, which allows
users to save only one specific parameter. It may be {pi, A,
B, all}, if the parameter that is to be saved is pi, A, B, or
all of the previous, respectively.