Added invoice export function and started implementation of unitofwork pattern (#43)

* complete read invoices from db

* wip

* wip

* wip

* wip

* wip

* wip

* wip

* wip

* updated nuget package spapi

* WIP

* wip, now test

* wip, jut need to fix tax inclusive line amounts not supported

* wip

* wip, before I f everything up

* no, it complies now, this is the one before I f everything up

* wip

* wip

* wip, logic ready for testing

* wip it builds!!!!

* wip tested, working, need to complete the gui section

* wip

* wip

* wip - created export invoice data delete, time for testing

* wip testing phase

* wip - delete function fully tested and working

* wip on to sorting out the issue with settlement invoices not tallying

* wip

* wip

* wip

* wip

* wip before I complete change the ReadInvoiceLineItem sections

* that appears to have worked, on with the main quest

* no it's doesn't work, saving before i remove the confusing cache system (just use a dictionary!!)

* wipping picadilli

* wip

* wip

* implemented uow on inovice export, now for testing

* wip

* wip all tested do invoice currency convertion fearure

* wip

* pretty much done so long as xero accepts the exported invoices

* Complete!

src/bnhtrade.Core/Model/Account/PurchaseInvoice.cs

@@ -8,6 +8,11 @@ namespace bnhtrade.Core.Model.Account
 {
     public class PurchaseInvoice
     {
+        // the purchase/sales invoice models are fragmented. The sales invoice inherits from an abstract invoice class. However, this purchase invoice
+        // model doesn't, it was setup in the early development to mirror an Amazon report.
+        // At some point I would like to correct this, i.e. have the purchase invoice model inherit from the abstract invoice class.
+        // this will take some unpicking
+
         public int PurchaseID { get; set; }
 
         public int PurchaseNumber { get; set; }