function decimalToLetters(value, aChar) {
        var letters = [];
        var aCode = aChar.charCodeAt(0);
        while (value > 0) {
            letters.unshift(String.fromCharCode((value - 1) % 26 + aCode));
            value = Math.floor((value - 1) / 26);
        }
        return letters.join('');
    }
converts a number to spreadsheet style letters
@param {number} value - the number
@param {string} aChar - the character representing the start of the sequence, e.g. 'A'
@returns {string} - the letters