Fix for the amsmath LaTeX package in the user manual.

It needs to be after the hyperref package, as hyperref clobbers a number of amsmath features.
Added all of the equations for the 'NS CPMG 2-site expanded' dispersion model to the relax manual.

These are essentially the source code modified to look good in LaTeX.
Fix for the 'NS CPMG 2-site expanded' model equations in the manual.