Added and modified convenience methods on the YearDate interface.

- Able to get the day of the week.
- Able to add an arbitrary number of days.
- Able to compare YearDate instances.
Completed season calculator.

- Trimmed the Season enum to only contain actual "season"s.
- Added a calculator class to work out the Gregorian date of Coptic new year.
Improved interface and calculator comments, removed useless method from YearlessDate.