relax SVN archive

The relax_fit C module unit tests now check if the parameter scaling is functional.

Created 2 unit tests for the target_functions.relax_fit relax C module.

These check the func() and dfunc() Python methods exposed by the module.

Big cleanup of estimate R2eff module.

This is to make the documentation more easy to read and understand.

task #7822(https://gna.org/task/index.php?7822): Implement user function to estimate R2eff and associated errors for exponential curve fitting.

The I0 and R parameter indices are now defined in the target_function/relax_fit.h header file.

This is to abstract the exponential curve parameter indices even more.

The parameter index is now passed into exponential_dI0() and exponential_dR().

This is for the relaxation curve-fitting C module so that the indices are not hardcoded.

Modified all of the exponential curve functions and gradients in the C target function module.

Instead of passing in the parameter vector, now the I0 and R values are passed in separately. This
is for greater code flexibility as the parameter order does not need to be known.

Changed the argument and variable names in the C code chi-squared gradient function.

Clean up of the target function C files (spacing fixes and removal of unused code).

Fixes for all of the exponential functions in target_functions/exponential.c.

The condition whereby Rx is zero is now setting the value correctly - the exponential will be 1, not
zero, hence the intensity and gradient values should not be zero.

Fixes for the target function chi-squared gradient C function.