Split out the interpolating in specific_analyses.relax_disp.data.plot_disp_curves() into separate function.
This is to prepare for a interpolating function for spin-lock offset rather than spin-lock field strength for R1rho models.
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].
