[Bug-gsl] Bessel Jnu domain error

[Cameron Williams]

In the documentation, there are no specifications for the values of nu in
the evaluation of bessel_Jnu. However after running the command for nu < 0,
a domain error was shot at me. After looking at the source code, it indeed
shows that a domain error is thrown when nu < 0. The cylinder Bessel
function however is defined when nu < 0 (for x > 0), so the documentation
should reflect that the routines only work when nu >= 0. This caused a bit
of a headache for me today and I figured I'd spare someone else from this.

Can the case of nu < 0 be easily implemented numerically? I am going to go
through the source code and see if I can get it to work for nu < 0 over the
next day or two.

Thanks!
Cameron Williams