Complete reworking of the 'NS R1rho 2-site' dispersion model.
The original code of Nikolai Skrynnikov and Martin Tollinger has been modified to match the
behaviour of Dmitry Korzhnev's cpmg_fit software. The equations from Korzhnev et al., JACS 2005
(http://dx.doi.org/10.1021/ja0446855) have been used for the initial magnetisation and the R1rho'
calculation. All equations have been added to the manual to clarify the model.