Add Product Weight to shopping_cart class

Add addition function in_cart_product_total_weight to obtain the weight of a Product in cart

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/zencart/zc-v1-series/232)
<!-- Reviewable:end -->