relax SVN archive

Shifted all of the model-free sample scripts into the new subdirectory sample_scripts/model_free.

Created the test_back_calc_new() relaxation data system test for the back calculation of new data.

Renamed test_back_calc2() to test_back_calc_specific().

Added the test_back_calc2() relaxation data system test for back calculation of specific data.

Added some data checks to the test_back_calc() system test of the relax_data.back_calc() user fn.

The ri_data_bc structure is not initialised by the model-free data_init() method.

The ri_data_bc data structure is now returned by get_data_name().

Fixes for the relax_data.back_calc() back end.

The user function was completely non-functional!

Created a very basic system test for the relax_data.back_calc() user function.

Improved the argument checking for the relax_data.back_calc() user function.