Modified back_calc_r2eff() to accept interpolated timepoints.
bug #22461(https://gna.org/bugs/?22461): NS R1rho 2-site_fit_r1 has extremely high chi2 value in systemtest Relax_disp.test_r1rho_kjaergaard_missing_r1.
Authored by: tlinnet 2014-08-22
Parent: [r25214]
Child: [r25216]
