add refund term in order_type when install
created initial methods for WC_Order_Refund class
created get_refunds() method for WC_Order class
fixed the get_refunds() method in WC_Order class
set order_type for refunds
created method to get the total amount already refunded
Merge pull request #5817 from claudiosmweb/improved-refund-and-order-classes

Improved refund and order classes