ITS version attribute

Hi Sebastian,

The current version of the ODD schema generates a mandatory
"its:version" attribute. That is correct for its usage at the rules
element, but not at a potential root element: imagine that a schema has
elements that can be root elements, but also other elements. In that
case "its:version" must not be mandatory.

Changing
<attDef ns="http://www.w3.org/2005/11/its" ident="version" usage="req">
to
<attDef ns="http://www.w3.org/2005/11/its" ident="version" usage="opt">
would break <its:rules> .

What do you think?

Felix