Special Functions.

• There are functions and functors for computing binomial coefficients.
• The ExponentialForSmallArgument functor uses truncated Taylor series to efficiently computes the exponential function for very small arguments.
• There are functions and functors for computing the factorial function and the logarithm of the factorial function. The LogarithmOfFactorialCached functor stores function values in a table.
• One can compute the Gamma function with LogarithmOfGamma.
• There are functions and functors for computing the harmonic number function.