Changed from a struct to a class

[BrandonRoehl]

• Provided lazy execution on sunrise sunset.
• class model instead of struct
• Made it so if you don't specify a date it will move dates if the time changes
  • This is done by having a getter and a private cache on the variables sunset and sunrise
• Can initialize an instance with a Zenith
• Backward compatible with the same interfaces and all test passing

I did these updates on a fork for a macOS application I have using this and decided to share the changes. If they don't fit with the current direction of the framework feel free to reject.