Add MIRI LRS dispersed spectrum 'PSF' sim

[mperrin]

As discussed with @skendrew and Achrene Dyrek, it could be useful to have a model for the dispersed LRS "PSF" (not really a PSF but a spectrum of PSFs). Achrene provided code for reading the dispersion law of LRS from CDP files. Use that to generate PSFs, offset, and sum.

Open research topic: better models for the cruciform in LRS data.

[mperrin]

Implemented externally of webbpsf, in https://github.com/mperrin/miri_lrs_fm