relax SVN archive

In the Python target function class Relax_fit_opt, the new func_sat(), dfunc_sat() and d2func_sat()
methods have been created as wrappers for the new C functions. These are aliased to func(), dfunc()
and d2func() in the __init__() method.

In the target_functions/exponential.c C file, the functions exponential_sat(),
exponential_sat_dIinf(), exponential_sat_dR(), exponential_sat_dIinf2(), exponential_sat_dR_dIinf()
and exponential_sat_dR2() have been created to implement the function, gradient, and Hessian for the
equation I = Iinf * (1 - exp(-R.t)).

In the target_functions/relax_fit.c file, the functions func_sat(), dfunc_sat(), d2func_sat(),
jacobian_sat() and jacobian_chi2_sat() have been created as duplications of the *_exp() functions,
but pointing to the exponential_sat*() functions and using Iinf instead of I0.