removed unneeded loop over sources in disaggregation calculator

This pr removes an unneeded loop over sources inside the disaggregation calculator (specifically inside the ``_arrange_data_in_bins`` method) and replaces it with a vectorized calculation.

I tested this patch using a realistic source model (based on area sources) and the computation time associated to the ``_arrange_data_in_bins`` dropped from 771.922 s to 122.675 s. You can find the full profile reports here: https://bugs.launchpad.net/oq-hazardlib/+bug/1174692