This bug was reported by Alex Hansen (viochemist_at_gmail_dot_com) and was
originally described at:
https://mail.gna.org/public/relax-users/2006-09/msg00026.html
(Message-id: <481156b20609251426r4a74f9f1p8fcf564354e61892@...>).
In addition to the fix of not skipping unselected residues described at
https://mail.gna.org/public/relax-devel/2006-09/msg00018.html
(Message-id: <7f080ed10609261109p2c7229e4ie02a894ac110dc3a@...>), I
have also added code to the 'self.num_instances()' model-free function to catch
the situation of 'self.determine_param_set_type()' returning None. If this
occurs, RelaxFault is raised!
