relax SVN archive

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.

Clean up of the end of the target_functions.relax_fit.dfunc() function.

The exponential curve-fitting gradient is now scaled by the scaling matrix.