Missing "reference" substatement in "revision" statement

[PrathibaJee]

For the proposal in the issue https://github.com/openBackhaul/core/issues/13 , need to include the reference sub-statement to the revision statement dated 2019-11-27

Solution : In the UML2YANG tool in the experimental branch , need to include statement "reference="ONF-TR-512, RFC 6020 and RFC 6087"" to the revision tag of 2019-11-27 in the /models/tools/pruning-and-refactoring/core-model/src/main/resources/xslt/prune-and-refactor-core-model.xslt

<revision xmi:type="OpenModel_Profile:Revision" xmi:id="onf-core-nf-revision-2019-11-27" date="2019-11-27" version="v1.4" description="Package equipment-specification has been deleted,&#xA;
       because of wrong key statements and overall grouping never applied.&#xA;" additionalChanges="" reference="ONF-TR-512, RFC 6020 and RFC 6087"/>