R1rho R_2 is defined as:
R_2 = R1rho / sin^2(theta) - R_1 / tan^2(theta) = (R1rho - R_1 * cos^2(theta) ) / sin^2(theta)
This is described more at:
http://wiki.nmr-relax.com/DPL94#Equation_-_re-writed_forms
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].
