fixed AiryDisk2D model
updated changelog
updated changelog
Updated AiryDisk2D model docs
defined module-level constant for AiryDisk2D model; fixed bug with peak-pixel amplitude
changed AiryDisk2DKernel input parameter from width to radius to match AiryDisk2D model
Updated test for AiryDisk2D model for non-unity amplitude
Added reference link for AiryDisk2D model
defined AiryDisk2D constant in the model class
Merge pull request #2093 from larrybradley/airy

fixed AiryDisk2D model