Could return complex values for real inputs if argument is negative

[heltonmc]

If called with negative arguments the routines could return imaginary values.

julia> besselj(0.4, 2.5)
0.24183102113825858

julia> besselj(0.4, -2.5)
0.07472989529876903 + 0.22999496849585185im

It appears the Julia standard is to return a domain error here unless called with complex. SpecialFunctions.jl will return error unless explicitly defined.

julia> using SpecialFunctions
julia> besselj(0.4, 2.5)
0.241831021138255

julia> besselj(0.4, -2.5)
ERROR: DomainError with -2.5:
`x` must be nonnegative.
Stacktrace:
 [1] besselj(nu::Float64, x::Float64)
   @ SpecialFunctions ~/.julia/packages/SpecialFunctions/NBIqR/src/bessel.jl:488
 [2] top-level scope
   @ REPL[5]:1

julia> besselj(0.4, -Complex(2.5))
0.07472989529876822 + 0.22999496849584836im

However, we do not yet support arguments with non zero imaginary components.

Edit: the answer in higher precision is `0.07472989529876900979841 +0.2299949684958517462472im