Replaced the method mfd.get_min_mag with a method mfd.get_min_max_mag

For the purpose of extracting the magnitude bins it is more convenient to have a method returning together
maximum and minimum magnitude. This is a small step towards https://bugs.launchpad.net/oq-engine/+bug/1279247

The tests pass: https://ci.openquake.org/job/zdevel_oq-hazardlib/60/