Converts an IEEE 754 compliant hexadecimal string into a JavaScript number.
@param {string} value - IEEE 754 compliant hexadecimal string to be converted
into a number.
@return {number} Decoded number.
function hex2num(value) {
  if (value === undefined) throw new Error('Value is undefined');

  var buffer = hex2buf(value);

  return readFromBuffer(buffer);
}