Maximum Average Subarray I

You are given an integer array `nums` consisting of `n` elements, and an integer `k`. Find a contiguous subarray whose **length is equal to** `k` that has the maximum average value and return _this value_. Any answer with a calculation error less than `10-5` will be accepted.   **Example 1:** **Input:** nums = [1,12,-5,-6,50,3], k = 4 **Output:** 12.75000 **Explanation:** Maximum average is (12 - 5 - 6 + 50) / 4 = 51 / 4 = 12.75 ``` **Example 2:** **Input:** nums = [5], k = 1 **Output:** 5.00000 ```   **Constraints:** - `n == nums.length` - `1 <= k <= n <= 105` - `-104 <= nums[i] <= 104`