relax SVN archive

Modified the structure.vectors() user function to fail if no vectors are found.

This saves a lot of sanity when debugging a user script which fails.

Fixes for the MC sim methods for deselected spins.

Removed the check for spin selection so that determine_model_type() works with nothing selected.

This is a partial response to bug #13259 (https://gna.org/bugs/index.php?13259) reported by
Pierre-Yves Savard (pierre-yves dot savard att bcm dot ulaval dot ca).

Clean up of the model_loop() checking.

Modified the model elimination print out to show the data pipe name.

This was picked up while trying to debug bug #13259 (https://gna.org/bugs/index.php?13259).

Corrected the name of a variable which was still as in the relax_fit code.

Corrected a few comments.

Added some references for relaxation dispersion equations.

Implemented the equation for back-calculation of R2eff for fast-exchange CPMG relaxation dispersion.

Started to modify the C modules by changing a variable for relaxation dispersion.

The code now goes further, but still much work is needed...

The change was:
relax_times -> cpmg_frqs