Add support for Bessel functions

[rnorris7756]

I would be willing to work on this. The C++ standard library does not have great support for Bessel functions, but the boost library has a good implementation here. What is this project's policy on including pieces of external libraries like boost?

[FrancescAlted]

Well, I think that evaluating Bessel functions inside numexpr is really too specific. numexpr comes with a virtual machine for evaluating expressions that has to lookup for many operators and functions already. Adding more functions is going to create a noticeable performance impact for all the other evaluations (even if they do not use Bessel functions). So I am -1 on this.

[rnorris7756]

Fair enough.

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