yoda: support a possible 'Reference data' qualifier

[GraemeWatt]

Similar to the existing 'Custom Rivet identifier' qualifier:

hepdata-converter/hepdata_converter/writers/yoda_writer.py

Lines 117 to 121 in 3f0330d

	# Allow the standard Rivet identifier to be overridden by a custom value specified in the qualifiers.
	if 'qualifiers' in table.dependent_variables[idep]:
	for qualifier in table.dependent_variables[idep]['qualifiers']:
	if qualifier['name'] == 'Custom Rivet identifier':
	rivet_identifier = qualifier['value']

the code should check for a qualifier with name 'Reference data' and value 'false'. In this case, the rivet_path should be defined with THY (theory predictions) instead of REF (reference data) and the graph.setAnnotation('IsRef', '1') line should be omitted.

Request from Christian Gutschow.