relax SVN archive

Bug fix for bug #13259 (https://gna.org/bugs/index.php?13259).

This bug was reported by Pierre-Yves Savard (pierre-yves dot savard att bcm dot ulaval dot ca).

The bug may not be completely gone yet.

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.