Introduce MeanOfDate reference frame

[rhannequin]

The mean coordinates of the date are related to the ecliptic or the mean equator and the mean equinox of the date. These are the geometric coordinates corrected for the precessional motion of the Earth’s rotation axis.

This introduces MeanOfDate reference frame, from Geometric, with precession of the date applied. By "date", we actually mean "instant", although the precession only changes slightly from one day to the next.

In Astronoby, the MeanOfDate reference frame is geocentric (like Astrometric). Mean of date coordinates don't have to be geocentric, but that's the design choice for Astronoby at the moment.

Like Geometric and Astrometry, the MeanOfDate reference frame offers equatorial and ecliptic coordinates, among with the Cartesian position and velocity, and the distance to the Earth's center.

instant = Astronoby::Instant.from_time(Time.utc(2025, 2, 7))
ephem = Astronoby::Ephem.load("de440s.bsp")

sun = Astronoby::Sun.new(instant: instant, ephem: ephem)

sun.mean_of_date.position.map(&:km).map(&:round)
# => Vector[110436790, -89767243, -38912451]

sun.mean_of_date.velocity.map(&:kmpd).map(&:round)
# => Vector[1747570, 1776918, 770305]

sun.mean_of_date.equatorial.right_ascension.str(:hms)
# => "21h 23m 34.6729s"

sun.mean_of_date.equatorial.declination.str(:dms)
# => "-15° 17′ 31.1747″"

sun.mean_of_date.ecliptic.latitude.str(:dms)
# => "+0° 0′ 0.4003″"

sun.mean_of_date.ecliptic.longitude.str(:dms)
# => "+318° 27′ 41.6839″"

sun.mean_of_date.distance.km.round
# => 147541931