Write a function that returns true or false depending on whether its input integer is a leap year or not.
The following rules determine if an integer is a leap year:
It is NOT a leap year if it is NOT divisible by 4.
It is a leap year if it is divisible by 4.
It is a leap year if it is divisible by 400.
It is NOT a leap year if it is divisible by 100 but NOT by 400.
For example:
1997 is not a leap year (not divisible by 4)
1996 is a leap year (divisible by 4)
1600 is a leap year (divisible by 400)
1800 is NOT a leap year (divisible by 4, divisible by 100, NOT divisible by 400)