relax SVN archive

Created the structure.translate user function front end.

Created a second system test for the structure.superimpose user function using 'fit to mean'.

This translates one model, superimposes it to the original, and checks to see if the structures
shift correctly.

Implemented the 'fit to mean' algorithm for the structure.superimpose user function.

Rearranged the back end of the structure.superimpose user function.

This is just to shift more code into the 'superimpose' module.

Created the Structure.test_superimpose_fit_to_mean system test.

This is to test the 'fit to mean' algorithm of the structure.superimpose user function.

Added a 3rd structure to the Structure.test_superimpose_fit_to_first system test and loosened the checks.

Implemented the back end of the structure.superimpose user function.

This only implements the 'fit to first' algorithm. The 'fit to mean' is yet to be implemented.

Shifted the structural API calc_displacement() method into the main module.

Created the arg_check.is_int_list() function for checking for lists of integers.

Implemented the front end of the structure.superimpose user function.