Add vignetting model

[dkirkby]

This issue is to implement an optional model of vignetting as a function of position in the focal plane.

Can vignetting be modeled as an achromatic multiplicative throughput factor that depends only on r? Should the model be specified via a tabulated data file or is there a standard parameterization that is flexible enough?

What is the definitive reference for the expected vignetting in DESI? Has the vignetting in eBOSS been measured?

[djschlegel]

DESI-2632 is the most detailed look at telescope vignetting. The chromatic effects will be quite small. A larger effect would be the vignetting as a function of the ADC position. This study uses a single ADC position.