feat(astrometry): add getLocalApparentSiderealTime

[michealroberts]

For accuracy, expose a getLocalApparentSiderealTime() that returns the true LST for an observer's location that takes into account nutational effects of the Earth's movement with respect to sidereal motion.

A location's Local Apparent Sidereal Time (LAST) is the right ascension of the zenith at some time, as measured against the true Earth equator and equinox (rather than the fictional mean equator and equinox, which ignore the Earth’s nutation).