Sunrise, sunset, moonrise and moonset.

[thomasing]

Proposed API functions:
time predict_sunrise(observer_, time, observation_)
time predict_sunset(observer_, time, observation_)
time predict_moonrise(observer_, time, observation_)
time predict_moonset(observer_, time, observation_)
All functions should use apparent elevation and correct for sun disk and moon disk diameter, such that sunrise happens at the moment the top of the sun disk is above the horizon. Functions should return the predicted time and fill observation pointer with observation data, if it is provided.

[ryeng]

I think we should have some more parameters to define which definition of sunrise/sunset is requested. See http://search.cpan.org/~jforget/Astro-Sunrise-0.95/lib/Astro/Sunrise.pm#DESCRIPTION for some definitions.

We can provide constants in header files for the most common ones.