docs: def _resolve_source(self, source):
          """Resolve source to filepath if it is a directory."""
          if os.path.isdir(source):
              sources = glob.glob(os.path.join(source, '*.sbml'))
              if len(sources) == 0:
                  raise ModelLoadError('No .sbml file found in source directory')
              elif len(sources) > 1:
                  raise ModelLoadError(
                      'More than one .sbml file found in source directory')
              return sources[0]
          return source