Revive Collide Mode

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/180)) by Nate Koenig (Bitbucket: [Nathan Koenig](https://bitbucket.org/%7Bc862cdd9-fcc8-4419-9fc7-e20db14b8fcb%7D/)).**

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Take a look at declined [pull request #288](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/288). It proposes a new tag for setting a link's collide mode.

Start by reading the archived issue and declined pull request #288 to recover the proposed collide-mode tag and why it was declined. Establish the intended format and behavior before locating the parser and description-file changes; done means an agreed design is implemented and its behavior is covered by validation or tests.

Written by the indexing model from the issue text.