add method checking for a shmita year

[Elyahu41]

I am surprised that this method is not already in the JewishCalendar class. It was the first method that I was looking for when I first started looking at the documentation. I believe that it is a good addition.

[NightScript370]

It just looks like a clone of isIsruChag()

[Elyahu41]

@NightScript370 you are right! Oops, I guess I forgot to actually write the code in the method. Thanks!