Update hwmatch.F for FxFx-Merging with Herwig6

This code is provided to use FxFx-Merging with Herwig6 inside CMSSW.

Josh Bendavid created the first version of this code, which was loosely based on the FxFx-Merging algorithm of the aMC@NLO authors and specially adopted to Higgs processes.
This adaption caused problems when used with other processes like ttbar. For that reason I ported it back to the original algorithm, see also http://amcatnlo.web.cern.ch/amcatnlo/mcatnlo_hwan_MLM.f