Added corrections for stars

[PovlAbrahamsen]

While the existing plugin worked well with sun, moon, and planet sights, it was not treating stars correctly. This update adds corrections for star sights.

1. It uses updated star positions from the SIMBAD Astronomical Database (http://simbad.u-strasbg.fr/simbad/)
2. Positions are corrected for proper motion and parallax.
3. Positions are corrected for frame bias, precession, and nutation.

New files:

• transform_star.cpp has been written by me, using equations from the US Naval Observatory Circular No. 179 (http://aa.usno.navy.mil/publications/docs/Circular_179.pdf)
• epv00.cpp comes from the SOFA library (http://www.iausofa.org/), with a wrapper function written by me.

To do: we should acknowledge the use of the SOFA function somewhere in documentation.

[rgleason]

That's great Povl. Thank you. We will add the proper credits you mention in the documentation.

[rgleason]

Ok that's done now. See https://opencpn.org/wiki/dokuwiki/doku.php?id=opencpn:opencpn_user_manual:toolbar_buttons:plugins:navigation:celestial_navigation#accuracy_of_data

Now we just need to figure out why in Windows version Opencpn bombs when entering the second sight.