Woocommerce Per Product Shipping 2.5.6

* Fix - Per product shipping rules order is not saved for variations after updating a product.
* Add - Support for local pickup shipping method.

* Add - Use shipping zones and instances to allow greater flexibility. Any existing configurations will use the legacy shipping method based on 2.2.14.
* Add - Split Per Product Shipping products into separate packages from other products.
* Add - Filter woocommerce_per_product_shipping_ship_via to allow per product shipping to use other (possibly free) shipping methods.

* Tweak - WC 3.6 compatibility.
* Fix - Updating rates defined on a product won't work