docs: def get_spectra_id(self, fn_id, retention_time=None, scan_nr=None):
          """Returns spectra id for spectra filename and retention time"""
          cursor = self.get_cursor()
          sql = 'SELECT spectra_id FROM mzml WHERE mzmlfile_id=? '
          values = [fn_id]
          if retention_time is not None:
              sql = '{0} AND retention_time=?'.format(sql)
              values.append(retention_time)
          if scan_nr is not None:
              sql = '{0} AND scan_nr=?'.format(sql)
              values.append(scan_nr)
          cursor.execute(sql, tuple(values))
          return cursor.fetchone()[0]