Add programmatic creation of SDF to DOM

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/182)) by Nate Koenig (Bitbucket: [Nathan Koenig](https://bitbucket.org/%7Bc862cdd9-fcc8-4419-9fc7-e20db14b8fcb%7D/)).**

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Add an API to the DOM classes (e.g. Model.hh, Link.hh, etc) that support programmatic creation of SDF worlds and models.

Start by reviewing the DOM class headers mentioned in the issue, including Model.hh and Link.hh, along with the related world and model classes. Define the API scope for constructing SDF worlds and models programmatically, then verify that the DOM supports creating those structures without requiring source XML.

Written by the indexing model from the issue text.