Fix for the relax_disp.r2eff_read_spin user function.

The offsets are now converted to ppm prior to finding the R2eff/R1rho key.
The relax_disp.plot_disp_curves user function can now support 150 sets per Grace graph.
The relax_disp.plot_disp_curves user function can now support 3000 sets per Grace graph.
System test for sequence read expanded to include assertions of correct data.

Work in progress for Support Request #3044, (https://gna.org/support/index.php?3044) - Load spins from SPARKY list.