Add Calculator Contract,

Add some checking for calculators,

src/Contracts/Calculator.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace Gloudemans\Shoppingcart\Contracts;
+
+use Gloudemans\Shoppingcart\CartItem;
+
+interface Calculator
+{
+    static function getAttribute(string $attribute, CartItem $cartItem);
+}