relax SVN archive

Added checks for the atomic information to the Structure.test_rmsd system test.

This demonstrates a failure of structure.add_atom user function when specifying different positions
for the different models.

Spun out the atomic_rmsd() and calc_mean_structure() functions into their own module.

They were previously in the generic_fns.structure.superimpose module but are now in the new
generic_fns.structure.statistics module.

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.