Update codestructure docs

Author: geraintpalmer

docs/Background/codestructure.rst

@@ -12,12 +12,25 @@ Ciw is structured in an object orientated manor.
    :alt: Code structure for Ciw.
    :align: center
 
-The classes that make up Ciw are:
+Ciw consists of 3 types of objects, Core, Input, and Optional.
+
+Core:
 
 - Simulation
 - Arrival Node
 - Exit Node
 - Node
 - Server
 - Individual
-- Data Record
+- Data Record
+
+Input:
+
+- Network
+- Service Centre
+- Customer Classe
+
+Optional:
+
+- State Tracker
+- Deadlock Detector