Provide access to Binning object in the Multiplexers

Closes #435 (closed)

Allows querying the binning of histograms in init e.g. like so

book(_h1, 1, 1, 1); // booked via ref data
book(_h2, 2, 1, 1); // booked via ref data
book(_h3, "h3", _h1.binning().edges<0>(), _h2.binning().edges<0>());