Bug [1954846] bessel_i(1/2,0) -> divide by zero error



I have implemented the suggested changes, which are reported to the bug
[1954846]. It works fine for all Bessel functions. The testsuite has no
problems.

With the additional tests we no longer get Lisp Errors, but a correct result.

besselexpand:true

bessel_j(1/2,0) --> 0
bessel_i(1/2,0) --> 0
bessel_y(1/2,0) --> infinity
bessel_k(1/2,0) --> infinity

bessel_j(-1/2,0) --> infinity
bessel_i(-1/2,0) --> infinity
bessel_y(-1/2,0) --> infinity
bessel_k(-1/2,0) --> infinity

When we use a Float number 0.5 or -0.5 for the order, we get the same results.
With besselexpand:false we get in all cases a noun form. 

Should I commit the changes to bessel.lisp?

Dieter Kaiser