Add warning for unbound indices in AO2D merging (#234)

Author: Raluca-Cruceru

docs/hyperloop/userdocumentation.md

@@ -336,6 +336,14 @@ When a wagon test finishes in warning, this means that the wagon will not be inc
   </div>
 
  * This is specific to tests with wagons set as ready for slim derived data. As the entire output is merged into one single file, there is a limit of 4000 MB for this. If exceeded, the user is advised to switch to standard derived data by unchecking the option “Ready for slim derived data” in the wagon edit view. Then a request for standard derived data train can be made.
+
+### 8. <a name="warning-unbound-indices"></a>Unbound indices detected in AO2D merging
+ 
+  <div align="center">
+    <img src="../images/warningUnboundIndices.png" width="40%">
+  </div>
+ 
+ * For derived data trains, it notifies the detection of unbound columns during AO2D merging. This means that one of the output tables which has been asked to be stored has index columns to tables which are not within the output. This usually points to a bad or broken data model definition and should be fixed. The only case where this is expected and not worrisome is linked derived data. For both slim derived data and standard derived data, the data model should be fixed.
 
 It is possible that a wagon test will produce multiple warnings. In that case, the same checks above will be done for each warning present, and the decision making regarding train submission will be done considering all the exceptions.