Returns the natural logarithm of the Gamma function evaluated at
x.'<br> Mathematically, the Gamma function is defined to be the integral from 0 to<br> Infinity of t^x exp(-t) dt. The implementation is copied, with extensions for<br> the reflection formula, from Numerical Recipes in C, section 6.1. The argument<br> x' must be positive. Full accuracy is obtained for x>1. For x<1, the
reflection formula is used. The computation is done using double-floats, and
the result is a double-float.