getChargeText(charge) {
    if (charge === 1) {
      return '+';
    } else if (charge === 2) {
      return '2+';
    } else if (charge === -1) {
      return '-';
    } else if (charge === -2) {
      return '2-';
    } else {
      return '';
    }
  }
Translate the integer indicating the charge to the appropriate text.
@param {Number} charge The integer indicating the charge.
@returns {String} A string representing a charge.