code: def get_source_by_alias(self, alias):
          
          for source in self.get(self._KEYS.SOURCES, []):
              if source[self._KEYS.ALIAS] == alias:
                  return source
          raise ValueError("Source '{}': alias '{}' not found!".format(self[
              self._KEYS.NAME], alias))
docs: Given an alias, find the corresponding source in this entry.
  
          If the given alias doesn't exist (e.g. there are no sources), then a
          `ValueError` is raised.
  
          Arguments
          ---------
          alias : str
              The str-integer (e.g. '8') of the target source.
  
          Returns
          -------
          source : `astrocats.catalog.source.Source` object
              The source object corresponding to the passed alias.