Reordered the output from return_offset_data(), interpolate_disp() and interpolate_offset(), to reflect the order of data type.
Made the input to return_grace_data_vs_disp(), return_grace_data_vs_offset the same().
Added the interpolate flag to return_grace_data_vs_disp() and return_grace_data_vs_offset(), to help return correct X-value.
Added the interpolate flag to return_x_y_point(), to help determine if "disp point" or "offset point" should be returned.
Added the "offset point" to return_x_y_point() function, to make it possible to plot against offset.
Cleaned up the return_grace_data_vs_offset() function, to use newly created return_x_y_labels() function.
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].
