Fix IQR statistic for shakemaps_from_quakeml workflow

While the min, max & median values in the statistics dict are as fractiles of g, the IQR value is not.

See: https://git.gfz-potsdam.de/shakemap/shakyground2/-/blob/master/shakyground2/workflows.py#L261

I would expect something like:

"IQR": np.exp(np.quantile(mean_shakemaps[imt], 0.75)) - np.exp(np.quantile(mean_shakemaps[imt], 0.25))