Modifications to the Relax_disp.test_m61_exp_data_to_m61 system test.
This is to fix bug #21869 (https://gna.org/bugs/?21869), the failure of this system test.
The number of Monte Carlo simulation for the 'R2eff' model has been increased from 3 to 25 using the
new exp_mc_sim_num argument to the dispersion auto-analysis. To keep the test fast, only a single
spin is optimised.
