Add deprecation to element description

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/26)) 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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We should be able to tag elements as deprecated, and the parser should print warnings to the console when deprecated tags are parsed.

Start by locating the element-description model and parser entry point responsible for reading tags. Completion means elements can be marked deprecated and the parser prints a console warning whenever a deprecated tag is parsed; verify both behaviors in the parser's existing coverage.

Written by the indexing model from the issue text.