Get mrrt coding scheme via external UID

In parsing an mrrt template we get all the ConceptSource's and compare their name and compare them (ignoring the case) against the name of the coding scheme in the mrrt template.

Replace this by the new method ConceptService.getConceptSourceByExternalUid(String)

passing in the coding scheme designator UID stored in the mrrt template.

Change needs to be done here

https://github.com/openmrs/openmrs-module-radiology/blob/master/api/src/main/java/org/openmrs/module/radiology/report/template/DefaultMrrtReportTemplateFileParser.java#L167