Aggregate knowledge around equinox calcuation so we can be more confident about a final implementation.

iran-ground-truth.json, Spring equinox times provided in Iran's time by calendar.ut.ac.ir, we like to have an implementation matching to it.

Implementations:

• https://github.com/xyproto/calendar/blob/master/equinox.go which it self is a port of http://jgiesen.de/astro/astroJS/seasons/seasons.js used on http://jgiesen.de/astro/astroJS/seasons/ Our current https://github.com/persian-calendar/equinox is based on it
• https://stellafane.org/misc/equinox.html this apparently matches better with Iran provided ones? (Dart & Go implementation from the same book)
• (feel free to add more)