include friction model parameter for ODE

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/96)) by Ying Lu (Bitbucket: [rosebudflyaway](https://bitbucket.org/%7B11ec131e-4af6-4228-beaf-167b1c8d1c2c%7D/)).**

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Add an element of `friction_model` to choose from pyramid, cone, and box friction model.
Commit [bb5e2bb](https://bitbucket.org/rosebudflyaway/sdformat/commits/bb5e2bb55e386697e8832971b869c7a0d3f063a1) added the element. This is corresponding to Gazebo pull request [1522](https://bitbucket.org/osrf/gazebo/pull-request/1522/cone-friction-model/diff#comment-6621619)