computes the armstrong numbers from xstart to xend inclusive
Armstrong number is a number that is the sum of its each digit raised to the power of the length of that number. Ex-1,2,3,4,153,370,371,407
For instance, 407 has number of digits 3 and the digits are 4,0 and 7
hence, the sum = 4^3 + 0^3 + 7^3 = 407
Rafi Ahmed (2019). Find Armstrong Numbers for Certain Range (https://www.mathworks.com/matlabcentral/fileexchange/72882-find-armstrong-numbers-for-certain-range), MATLAB Central File Exchange. Retrieved .