Add two new calculators

- MeeusCalculator - a calculator that implements the higher accuracy Jean Meeus algorithmn (the NOAACalculator uses the low accuracy implementation)
- SPACalculator - a higher accuracy calculator

At this point there are no plans to add an implementation of the JPL DE ephemerides-based algorithms. The JPL Development Ephemeris files — binary tables (SPK/.bsp) giving high-accuracy positions of the Sun, Moon, and planets over a span of years — require loading 114 MB for the "short-term" 1550–2650 calculations, and ~2.6 GB for the full ~13200 BC–17191 AD span. The JPL ephemerides exceed SPA's accuracy at the milliarcsecond level, but for sunrise/sunset the resulting difference is a fraction of a second and is dominated by atmospheric refraction, whose standard model is itself uncertain by a much larger margin (on the order of tenths of an arcminute depending on local weather). The improvement is therefore imperceptible in practice and not worth the added size and complexity.

Author: KosherJava