static number2alpha(n) {
    if (!Number.isInteger(n)) {
      throw new Error('Invalid number');
    }
    if (n < 0) {
      throw new Error('Number cannot be negative');
    }
    const alphalength = XTableUtils.alphachars.length;
    if (n < alphalength) {
      return XTableUtils.alphachars[n];
    }
    let result = '';
    let x = n;
    while (x >= 0) {
      const currentMod = x % alphalength;
      const currentValue = XTableUtils.alphachars[currentMod];
      result = currentValue + result;
      x = (x - currentMod) / alphalength - 1;
    }
    return result;
  }
Given a decimal number, transform it to alpha format.
@param {Number} n Number in decimal format.
@return {String} Number in alpha format.