LOAD/SAVE/CHECK COMPATIBILITY OF HYPOS

Some learning algorithms take a long time to
run, and so it might be worth saving their
results. The following two optional functions
follow our standard *r_error_expo convention
to report loading/saving problems:
r_error_expo is set to NULL if the operation
succeeds, and is otherwise set to a message
explaining the problem.

   char *mk_load_xxx_hypo(char *filename,
                          expo **r_error_expo)
   void save_xxx_hypo(char *hypo,
                      char *filename,
                      expo **r_error_expo)

A freshly loaded hypo might be incompatible
with the current dataset or the current
attributes of interest. The next (optional)
function checks that. If the hypo is
compatible, then this function sets
*r_error_expo to NULL. If the hypo is
incompatible, this function sets
*r_error_expo to an explanation of the
problem.  If this function is not provided,
unfortunate and potentially catastrophic
errors may occur if the hypo is incompatible.

   bool xxx_hypo_compatiblep(examples *exams,
                            ivec *in_att_nums,
                            ivec *out_att_nums,
                            char *hypo,
                            **r_error_expo)