relax SVN archive

The 'chi2()' C function is now declared in the 'func()' function as returning a double. Previously
it was interpreting it the result as an int!

The exponential back calculation function has been debugged. The function is I0 * exp(-t * Rx) and
not I0 * exp(-t / Rx)!

In 'maths_fns/c_chi2.c' the function 'square()' has been defined.

The makefile 'maths_fns/Makefile' has been modified so that changes to 'maths_fns/relax_fit.h' will
now cause recompilation of all C code.

The spectral density function 'num_instances()' in 'specific_fns/jw_mapping.py' has been debugged.

In 'specific_fns/relax_fit.py', the function 'ave_and_sd()' has been renamed to 'mean_and_error()'.
The user function 'mean_and_error()' has been added to 'prompt/relax_fit.py'. Previously this was
being called by the minimise function of 'specific_fns/relax_fit.py', ie both during the grid search
and the minimisation. This function has to be called prior to optimisation.

For Monte Carlo simulations, the function 'back_calc_I()' has been added to 'maths_fns/relax_fit.c'.
This simply converts the C external variable 'back_calc[]' to a Numeric array and returns it. The
code isn't fully functional yet. In addition, in the file 'specific_fns/relax_fit.py', the
functions 'num_instances()', 'return_data()', 'set_selected_sim()', and 'sim_pack_data()' have been
added for MC sims.

The error function 'RelaxNoMfModelError' has been changed to 'RelaxNoModelError' to generalise it
for both model-free analysis and relaxation curve fitting.

The function 'sim_set_selected()' has been renamed to 'set_selected_sim()' in
'specific_fns/model_free.py'.