Numerical issue with obscured airy disk

[sefffal]

The flux exactly on-axis is incorrect:

julia> a = airydisk(
           x = 40.5, y=40.5, fwhm=10, ratio=0.2,amp=1)
#24 (generic function with 1 method)

# Slightly off axis
julia> a(40.51,40.51)
0.9999945790370132

# Exactly on axis:
julia> a(40.5,40.5)
1.0850694444444444

The flux exactly (same float, not float + eps) on axis is wrong by 8.5%.

[sefffal]

I believe the bug is here @mileslucas:

PSFModels.jl/src/airy.jl

Line 46 in feaa43c

iszero(r) && return amp / (1 - ratio^2)^2 + background

The short-circuit for if exactly on axis (r==0) doesn't account for the ratio.