replace sum() with + in calibrate_thresholds() for two-sample case

[zabore]

There is a code mistake on lines 365 and 369 in calibrate_thresholds()
Need to replace prop_stopped_null = mean(sum(n0_null, n1_null) < sum(N)) with prop_stopped_null = mean((n0_null + n1_null) < sum(N)) on line 365 and prop_stopped_alt = mean(sum(n0_alt, n1_alt) < sum(N)) with prop_stopped_alt = mean((n0_alt + n1_alt) < sum(N)) on line 369.