This is make it possible to switch between the interpolating functions, when plotting R1rho graphs.
This is necessary to produce the R2 as function of effective field in rotating frame w_eff, and to produce
R1rho as function of theta, when ramping the spin-lock offset.
These graphs can be seen at:
http://wiki.nmr-relax.com/Matplotlib_DPL94_R1rho_R2eff
sr #3124(https://gna.org/support/?3124): Grace graphs production for R1rho analysis with R2_eff as function of Omega_eff.
sr #3138(https://gna.org/support/?3138): Interpolating theta through spin-lock offset [Omega], rather than spin-lock field strength [w1].
