Don't require signal processes to have a mass

[ajgilbert]

When creating the mapping rules should check if mass is convertible to float and if not don't put $MASS in the pattern:

HiggsAnalysis-HiggsToTauTau/CombineHarvester/CombineTools/src/CombineHarvester_Datacards.cc

Lines 391 to 399 in b3225ac

	CombineHarvester ch_signals =
	std::move(this->cp().bin({bin}).signals().histograms());
	auto sig_proc_set =
	ch_signals.SetFromProcs(std::mem_fn(&ch::Process::process));
	for (auto sig_proc : sig_proc_set) {
	mappings.emplace_back(sig_proc, bin, bin + "/" + sig_proc + "$MASS",
	bin + "/" + sig_proc + "$MASS_$SYSTEMATIC");
	}
	}