hyperbolic cosine inverse
t = acosh(x)
each is a real or complex vector or matrix.
t has the sizes of x.
acosh(x) returns t such that
cosh(t)==x and real(t)>=0.
-t are always other possible answers.
For real input numbers -1 < x < 1,
real(t)==0 and imag(t)
belongs to ]0, %pi[.
For complex numbers x,
imag(t) belongs to [-pi, pi]
and any t + k*%pi*%i with integer k are other
possible answers.
