enable Session.__init__() to be provided with existing instance or book

bindings/python/gnucash_core.py

@@ -210,7 +210,12 @@ class Session(GnuCashCoreClass):
         you don't need to cleanup and call end() and destroy(), that is handled
         for you, and the exception is raised.
         """
-        GnuCashCoreClass.__init__(self, Book())
+        if instance is not None:
+            GnuCashCoreClass.__init__(self, instance=instance)
+        else:
+            if book is None:
+                book = Book()
+            GnuCashCoreClass.__init__(self, book)
 
         if book_uri is not None:
             try: