Expose planet orbital periods

This release adds a new function PlanetOrbitalPeriod that returns the average number of days it takes a planet to orbit the Sun. This number should be considered an approximation. First of all, the Sun is wobbling around the Solar System Barycenter because of gravitational pull from the major planets Jupiter, Saturn, Uranus, and Neptune. Therefore, the concept of the exact moment a planet finishes one orbit "around the Sun" is vague. Secondly, the planets are all pulling on each other, so their orbits do not have a fixed shape. This adds to the vagueness of the orbital period. However, some applications may benefit from this rough concept of orbital period.