The idea is to recursively select each element and see if it can be used to reach to the target.
1. If start_index >= length of the array, return true when the target is zero, false otheriwse.
2. If the element at the start index is equal to must_have, recurse by subtracting that element from the target. i.e. return groupSumWithNum(start_index+1,arr,must_have,target-must_have);
3. Include the first number and check the remaining numbers. i.e. groupSum(start_index+1,arr,target-arr[start_index]))
4. Exclude the first number and check the remaining numbers. i.e. groupSum(start_index+1,arr,target)
