relax SVN archive

Shifted the XH vector calculation from the xh_vector() structural API method to the vector() fn.

This calculation can be done independently from the type of the structural data object.

The generate_spin_id() fn is now being called by the vectors() fn.

The generate_spin_id_data_array() fn is now called in the read() method.

Fixed the call to generate_spin_id() in the return_spin_from_index() function.

Renamed generate_spin_id() to generate_spin_id_data_array(), and created a new generate_spin_id().

The generate_spin_id_data_array() function generates the spin_id string from a data array whereas
generate_spin_id() generates the spin_id string from the argument values passed into it.

Added the structural API atom_loop() call to the vectors() function.

Renamed the structural API base class method num_str() to num_models().

Fixed the angles.py script used in the system tests.

The heteronuc arg to structure.vectors() has been removed.

Modified the structural API stub method atom_loop() to accept the model_num_flag arg.

Modified the Scientific Python API atom_loop() method to return the model number when asked.