relax SVN archive

Modified the structure.add_atom user function to allow the position argument to be a rank-2 array.

This allows a different coordinate for each model to be specified.

Bug fix for the arg_check.is_float_matrix() function for when the dimensions are not given.

Added the 'model_num' argument to the structure.add_model user function.

Modified the structure.add_model calls in the Structure.test_rmsd system test to include model nums.

Added some more checks to the Structure.test_rmsd system test.

Implemented the structure.add_model user function.

The structural API num_molecules() method can now handle no data being present.

Created the Structure.test_rmsd system test.

This test checks the currently unimplemented structure.add_model and structure.rmsd user functions.

Added a printout to structure.delete for when no structures are present.

Fixes for the structure.delete user function.

This can now handle no data being present, the spin and interatomic data containers are now have
their structural data properly deleted, and printouts have been added so the user knows what has
happened.