relax SVN archive

Removed some redundant code.

Started to redesign add_struct() to remove the struct_index arg and convert to the new model design.

The API base class function get_model() has been added to return or create the desired model. The
add_struct() method has been renamed to add_molecule(). Much work remains to be done.

Support for models vs. molecules and the elimination of struct_index in bond_vectors().

Fix for the struct_index arg unit test of structure.vectors().

The arg is now called 'model'.

Added a verbosity flag to parse_token() so that print outs are suppressed by default.

Docstring fixes for vectors().

Support for models vs. molecules and elimination of struct_index in vectors().

Modified the prompt interface of the structure.vectors() user function.

The struct_index arg is dead! Long live the model arg!

Corrected mistakes in English.

Corrected mistakes in English.