sumRegion(row1, col1, row2, col2) {
    let sum = 0;
    for (let i = row1; i < row2 + 1; i++) {
      for (let j = col1; j < col2 + 1; j++) {
        sum += this.matrix[i][j];
      }
    }
    return sum;
  }
m = row2 - row1; n = col2 - col1
Time O(m*n) | Space O(1)
@param {number} row1 
@param {number} col1 
@param {number} row2 
@param {number} col2
@return {number}