ASINH

     Return the inverse hyperbolic sine of the argument

     The inverse hyperbolic sine is used for the calculation of asinh
     magnitudes, see Lupton et al. (1999, AJ, 118, 1406)

     result = asinh( x)

     X - hyperbolic sine, numeric scalar or vector or multidimensional array
        (not complex)

     result - inverse hyperbolic sine, same number of elements as X
              double precision if X is double, otherwise floating pt.

     Expression given in  Numerical Recipes, Press et al. (1992), eq. 5.6.7
     Note that asinh(-x) = -asinh(x) and that asinh(0) = 0. and that
     if y = asinh(x) then x = sinh(y).

     Written W. Landsman                 February, 2001
     Work for multi-dimensional arrays  W. Landsman    August 2002
     Simplify coding, and work for scalars again  W. Landsman October 2003

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