relax SVN archive

Printout improvements for the structure.rmsd user function.

Updated the generic_fns.structure package __all__ variable to include the statistics module.

Created a new float_object argument type which is used by the 'pos' argument of structure.add_atom.

A new arg_check.float_object() function has been created to handle any float object greater than
rank-0.

Fixes for the Structure.test_rmsd system test - it now passes.

Implemented the structure.rmsd user function.

Docstring addition for the generic_fns.structure.statistics.atomic_rmsd() function.

Fix for the structure.add_atom user function for the internal structural object.

The atomic positions for each model are now correctly set.

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.