These changes complete task #3838 (https://gna.org/task/?3838). The details of the changes were
discussed at https://mail.gna.org/public/relax-users/2006-10/msg00002.html (Message-id:
<7f080ed10610032153o4571935bq138d2739f55ca3bd@...>).
The only difference between the post proposing the changes and the actual changes is that the
proposed user function 'pdb.set_vectors()' is actually now called 'pdb.vectors()'.