Fix for bug #15050 (https://gna.org/bugs/index.php?15050).
This bug was submitted by Tiago Pais (https://gna.org/users/tpais).
The _determine_model_type() method has been modified to return None when all parameters are fixed
(or if there no selected spins and no diffusion tensor). The model-free minimisation method has
also been modified to handle this.
