relax SVN archive

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.

The Structure.test_superimpose_fit_to_first system test is now testing the backbone heavy atom fit.

Created the Structure.test_superimpose_fit_to_first system test.

The structure.superimpose user function doesn't exist yet, but all the necessary algorithms are
already in relax.