Function doomsday::is_leap

pub fn is_leap(year: usize) -> bool

Return true for given years which are defined to be leap years.

This is designed to help account for the fact there are not in fact a perfect 365 days in each year. Where, a day is defined to be a complete rotation of the earth around it's axis (not to be confused with a solar day), and a year is defined to be the time it takes the earth to complete a trip in it's orbit around the sun.