code: def get_source_by_name(self, name):
          
          srcs = self.get_sources_by_name(name)
  
          if len(srcs) == 1:
              return srcs[0]
          elif len(srcs) == 0:
              raise Exception('No source matching name: ' + name)
          elif len(srcs) > 1:
              raise Exception('Multiple sources matching name: ' + name)
docs: Return a single source in the ROI with the given name.  The
          input name string can match any of the strings in the names
          property of the source object.  Case and whitespace are
          ignored when matching name strings.  If no sources are found
          or multiple sources then an exception is thrown.
  
          Parameters
          ----------
          name : str 
             Name string.
  
          Returns
          -------
          srcs : `~fermipy.roi_model.Model`
             A source object.