restructuration of the documentation

doc/source/types.rst

@@ -0,0 +1,25 @@
+********
+OBITypes
+********
+
+    
+.. image:: ./images/OBITypes_UML.png
+:download:`html version of the OBITypes UML file <UML/OBITypes_UML.class.violet.html>`
+
+.. note::
+    All OBITypes have an associated NA (Not Available) value.
+    We have currently two ideas for implementing NA values:
+    
+    		- By specifying an explicit NA value for each type
+    		- By adding to each column of an OBIDMS a bit vector
+    		  indicating if the value is defined or not.
+
+
+
+.. toctree::
+   :maxdepth: 2
+
+   The elementary types <elementary>
+   The containers <containers>
+
+