code: def find_external_files(self, run_input_dir):
          
          beam_file, geo_file, mat_file, _ = self.input_files
  
          # check for external files in BEAM input file
          external_beam_files = self._parse_beam_file(beam_file, run_input_dir)
          if external_beam_files:
              logger.info("External files from BEAM file: {0}".format(external_beam_files))
          else:
              logger.debug("No external files from BEAM file")
  
          # check for external files in MAT input file
          icru_numbers = self._parse_mat_file(mat_file)
          if icru_numbers:
              logger.info("External files from MAT file: {0}".format(icru_numbers))
          else:
              logger.debug("No external files from MAT file")
          # if ICRU+LOADEX pairs were found - get file names for external material files
          icru_files = []
          if icru_numbers:
              icru_files = self._decrypt_icru_files(icru_numbers)
  
          # check for external files in GEO input file
          geo_files = self._parse_geo_file(geo_file, run_input_dir)
          if geo_files:
              logger.info("External files from GEO file: {0}".format(geo_files))
          else:
              logger.debug("No external files from GEO file")
  
          external_files = external_beam_files + icru_files + geo_files
          return [os.path.join(self.input_path, e) for e in external_files]
docs: Scan all SHIELDHIT12A config files to find external files used and return them.
          Also change paths in config files to match convention that all resources are
          symlinked in job_xxxx/symlink