relax SVN archive

Created the Structure.test_rmsd_ubi system test to better check the structure.rmsd user function.

This uses the truncated ubiquitin ensemble in the test suite shared data directories. The RMSD
matches the VMD 1.9.1 output.

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.