this function approximates e-Euler's number
e is the estimated value whereas k is the smallest value at which
the function ceases and the approximation differs from exp(1)
(i.e., the value returned MATLAB’s built-in function) by no more than the
positive scalar, delta.
Note: e = summation(1/k!), for k = 0:inf.
Mahdi Zolfaghari (2020). Estimetaion_e (https://www.mathworks.com/matlabcentral/fileexchange/68236-estimetaion_e), MATLAB Central File Exchange. Retrieved .