Day 13 : Order class's calculate method is not pure

[havanagrawal]

Calling calculate on a given Order should return the same value, regardless of how many times it is called.

The current implementation will return a new, increased value each time.

https://github.com/sunnypatel165/31DaysOfRefactoringInJava/blob/master/src/com/sunnypatel/daysofrefactoringjava/day13/extractmethodobjects/problem/Order.java#L20-L30