code: function check_STEP_file(filename, callback) {
  
      "use strict";
  
      // "ISO-10303-21;"
      // "HEADER;"
      var stream = fs.createReadStream(filename, {flags:"r"});
  
      var fileData = "";
      stream.on('data', function (data) {
          fileData += data;
  
          // The next lines should be improved
          var lines = fileData.split("\n");
  
          if (lines.length >= 2) {
  
              stream.destroy();
              if (!pattern_ISO_10303_21.test(lines[0])) {
                  my_callback(new Error("this file is not a STEP FILE : ISO_10303_21 missing"));
              } else {
                  my_callback(null, lines[0]);
              }
          }
      });
      stream.on('error', function () {
          my_callback('Error', null);
      });
  
      stream.on('end', function () {
          my_callback('File end reached without finding line', null);
      });
  
      var callback_called = false;
  
      function my_callback(err, data) {
          if (!callback_called) {
              callback_called = true;
              callback(err, data);
          }
      }
  }
docs: return true if the file is a correct STEP file
  a correct STEP file starts with
  ISO-10303-21;
  HEADER;
  
  @param filename
  @param callback