Goal: Use Python to implement Product Sum
Write a function that takes in a "special" array and return its product sum.
A "special" array is a non-empty array that contains either integers or other "special" arrays. The product sum of a "special" array is the sum of its elements, where "special" arrays inside it are summed themselves and then multiplied by their level of depth.
The depth of a "special" array is how far nested it is. For instance, the depth of [] is 1; the depth of the inner array in [[]] is 2; the depth of the innermost array in [[[]]] is 3.
Therefore, the product sum of [x,y] is x+y; the product sum of [x, [y,z]] is x + 2 * (y+z); the product sum of [x,[y,[z]]] is x + 2 * (y + 3z)