SciX ID included in output

[ehenneken]

This PR represents the work done to include the SciX ID in the output of the service. In order to achieve this goal, the following was done

• Update config.py to include the Solr field scix_id in REFERENCE_SERVICE_QUERY_FIELDS_SOLR so that this field is always present when results (i.e. potential matches) are retrieved from Solr
• Whether the /text or /xml endpoint is used to submit reference data, the actual processing always ends up in the function solve_for_fields. Here the contents of the Solr field scix_id need to be passed on when creating the Solution instance sent back
• The Solution class needs to be augmented to support the inclusion of the SciX ID through the scix_id attribute. The string representation of class instances __str__ needs to be updated to include the value of this attribute. This is because the result of the matching process is passed on as str(solve_reference(Hypotheses(parsed_ref))) (the resolve_reference returns a Solution instance).
• The response to be sent back is generated in the format_resolved_reference function. This function parses the text string generated by stringifying the Solution instance and creating a JSON structure with the results of the matching