Change README and visinfo to require topic files to be listed in view…

…points

Documentation/README.md

@@ -223,7 +223,7 @@ The visualization information file contains information of components related to
 
 ### TopicFiles
 
-Parent topic objects specify a list of model files in their `Header.Files` element. These files contain an `Id` attribute, which can be used to optionally reference a subset of all the topic files for a specific viewpoint. If the viewpoint does contain entries in it's `TopicFiles` element, then only those files referenced there should be loaded when visualizing the viewpoint. Otherwise, all files from the topic should be loaded.
+Parent topic objects specify a list of model files in their `Header.Files` element. These files contain an `Id` attribute, which are used to reference a subset of all the topic files for a specific viewpoint. To make viewpoints immutable, e.g. in the case of later added topic files, each viewpoint is required to list all their file references. Only those files referenced In a viewpoints `TopicFiles` should be loaded when visualizing the viewpoint.
 
 ### Components
 

Schemas/visinfo.xsd

@@ -7,9 +7,7 @@
  </xs:annotation>
  <xs:complexType>
  <xs:sequence>
- <!-- If one or more TopicFile elements are present, then this means that the
- viewpoint is only supposed to load the given file references. Otherwise, the files
- to be loaded in the viewpoint are the same as the Files list in the parent topic.-->
+ <!-- The TopicFiles element lists the model files this viewpoint is supposed to load.-->
  <xs:element name="TopicFiles" type="TopicFiles"/>
  <!--
  Although plural, 'Components' is not a collection
@@ -112,7 +110,8 @@
  </xs:simpleType>
  <xs:complexType name="TopicFiles">
  <xs:sequence>
- <xs:element name="TopicFileId" type="TopicFileId" minOccurs="0" maxOccurs="unbounded"/>
+ <!-- A viewpoint needs to list it's referenced files, so at least one file must be present. -->
+ <xs:element name="TopicFileId" type="TopicFileId" maxOccurs="unbounded"/>
  </xs:sequence>
  </xs:complexType>
  <xs:complexType name="TopicFileId">