Y = expint(X)
The exponential integral computed by this function is defined as
Another common definition of the exponential integral function is the Cauchy principal value integral
which, for real positive x, is related to
Y = expint(X) evaluates
the exponential integral for each element of
 Abramowitz, M. and I. A. Stegun. Handbook of Mathematical Functions. Chapter 5, New York: Dover Publications, 1965.