Progress Bar Smoothing

[alexdaniel654]

The ETA output by the relaxometry progress bars is pretty unstable (especially when calculating maps of a masked image), try using the smoothing parameter to make this a bit more accurate.
e.g.

results = list(tqdm(map(fit_signal,
                        model.signal_list,
                        model.x_list,
                        model.p0_list,
                        model.mask_list,
                        [model] * len(model.signal_list)),
                    total=len(model.signal_list),
                    smoothing=0.9))