Fix for bug #20613 (https://gna.org/bugs/?20613), the failure of the diffusion tensor PDB creation.
This was reported by Angelo Miguel Figueiredo <am dott figueiredo att fct dot unl dot pt>.
The problem was that the diffusion tensor PDB representation structure.create_diff_tensor_pdb user
function was being called even when the local tm global model was selected. This naturally failed
as there is no global diffusion tensor. Now this user function is avoided for the local tm global
model.
