Deprecating sdf elements

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/98)) by Steve Peters (Bitbucket: [Steven Peters](https://bitbucket.org/%7B2ccfed09-18b8-4921-8d58-15ef01092802%7D/), GitHub: [scpeters](https://github.com/scpeters)).**

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It looks like setting `required="-1"` in an element description is [interpreted as a deprecation flag by the sdformat parser](https://github.com/osrf/sdformat/blob/31338eb0c6a53ff02cb49f9ccf31e13aa60cc769/src/parser.cc#L447). It also looks like the element will be skipped, so marking something as deprecated will provide a console message but will render the element unusable.

Perhaps we should add a new sdformat description attribute for deprecation? This way the value can still be parsed, while providing the warning message about deprecation.