The idea is to recursively select each element and check if it can be used to reach to the target.
1. If start_index >= length of the array, return true when target is zero, false otheriwse.
2. Include the first number and check the remaining numbers. i.e. groupSum(start_index+1,arr,target-arr[start_index]))
3. Exclude the first number and check the remaining numbers. i.e. groupSum(start_index+1,arr,target)
