Add Tekufa methods

[Elyahu41]

Here are 3 methods that can help people calculate when one of the 4 tekufas is every year. I did not know what to name the method that returns an int based on which tekufa it is, so I named it getTekufaType. Let me know if you want to change that name. The only other thing I changed is the javadoc in the getMoladAsDate method to link to the Date class since it seemed nicer.