relax SVN archive

Another bug fix for the create_vector_dist() function.

This time the atom number was not being incremented for chain B so that there are multiple atoms of
the same number in the PDB file.

Bug fix for the create_vector_dist() function.

The chain B of symmetry vectors was exactly the same as chain A and not mirrored.

Bug fixes for the PyMOL cartoon() method.

As with the open_pdb() method, cartoon() was not handling the structural object correctly and was
not handling multiple structures.

Bug fixes for the PyMOL open_pdb() method.

This method was not handling the structural object properly.

Bug fix for the create_vector_dist() function.

A structure needed to be added to the internal structural object.

Converted the symmetry flag to a bool.

A number of fixes for the create_vector_dist() function.

Added the missing spin_loop() import.

Added a system test for the creation of an XH bond vector distribution.

This test mimics the script used to create figure 7.8 of my PhD thesis
(http://eprints.infodiv.unimelb.edu.au/archive/00002799/).

Fix for bug #12670 (https://gna.org/bugs/index.php?12670).

This bug was reported by Michael Funk (mifu att nmr dot mpibpc dot mpg dot de).

The problem was that the relaxation curve-fitting return_error() method was returning the global
cdp.sigma_I error structure - which does not exist for all error types - when it should have been
returning the spin specific error structure.