Pathway to support 4D Comrade Intensity Map and SkyModels

[kazuakiyama]

VLBISkyModels.jl now has a major development project to fully support 4D images, which will finally open up the opportunity for our package to more instantaneously simulate multi-frequency, time-dependent scattering effects. As of Oct, 2024, VLBISkyModels.jl implements 4D IntensityMap and visibility_point function, but higher-level methods yet don't support them (see #26). The checklist is found here.
EHTJulia/VLBISkyModels.jl#53

This posting lists the anticipated tasks we should do once we are ready.

Diffractive Scattering
AbstractScatteringModel is not sensitive to time and frequency. But the kernel behavior is frequency dependent.

• update visibility_point method for ApproximateScatteringKernel and ExactScatteringKernel
  (see 4D image support checklist VLBISkyModels.jl#53)
• check VLBISkyModels's convolve (for IntensityMap) and convolved (for AbstractModel) for updated visibility_point methods
• update ensembleaverage method for AbstractScatteringModel using convolve and convolved methods
• not mandated but ideally automate the setting for νref for kernelmodel method

Refractive Scattering
The current implementation of RefractivePhaseScreen and PhaseScreenPowerLaw is insensitive to frequency, and has fields (screen velocity) necessary to support the time-domain simulations

• update scatter_image function. Basically, the procedure after the diffractive scattering needs to be looped for each set of time and frequency.