Added function to calculate rotating frame paramters for lib/nmr.py.
This function is called several times in specific_analyses/relax_disp/data.py by plotting functions.
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].
