bessel[IJKY] functions

[bearophile via Digitalmars-d-announce]

Ilya Yaroshenko:

> Ported from r-project:
> http://code.dlang.org/packages/bessel

If you like to write the full path like "std.math.isNaN", then 
you can also use static imports.

Perhaps it's worth adding pre/post-conditions to the functions.

The indentations probably have a mix of tabs and spaces and seem 
messed up.

Is gamma_cody better/worse than std.mathspecial.gamma?

Bye,
bearophile