<lrml:inScope> should not have content, only attributes

Currently the <inScope> element is allowed to have content which is either <lrml:Statements> or one of the LegalRuleML Statement elements. The original intent was that the Node element would carry a @keyref attribute to refer to the Statement(s) definition that occurs in the proper place. However, it would be necessary to define a different element type to enforce that only empty Statement(s) appear within <inScope>. But there is no usecase for this empty element that could not be handled just as well by an empty <inScope> tag with @keyref attribute.