Changes

* Fix for all edge case [[No Rex]] failures of the [[NS CPMG 2-site expanded]] model. This uses the no exchange checking idea, modified to function in the relax trunk, from http://article.gmane.org/gmane.science.nmr.relax.devel/5847. This is importantly on line 1 of the function. The recently introduced set of 7 unit tests comprehensively showing these failures now all pass.

* Important bug fix for the structure.delete user function when multiple models are present. This is to fix bug #22070 (https://gna.org/bugs/?22070), the failure of the structure.superimpose user function after deleting atoms with structure.delete. The problem is that structure.delete was removing the atoms from the first model but none of the others. This is because it was using the structural object atom_loop() method to find the atoms to be deleted, but this method operates on the first model. So when the second model is reached, the atoms are already gone.