override arbitrary parts of included models

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/128)) by Dirk Thomas (Bitbucket: [Dirk Thomas](https://bitbucket.org/%7Ba8effe61-90f2-4949-89a8-9c6972071de7%7D/), GitHub: [dirk-thomas](https://github.com/dirk-thomas)).**

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Currently only the `pose` and the `static` flag can be overridden for an included model. It would be great if this would be possible for any element.

An example use case would be to override the visual material and give the model a different color.

Without this feature it is not possible to reuse an existing model without either:

* converting it into a template (which you might not have write access to if it is a third-party model)
* copy-n-paste it and then modify it (which is kind of sad)

The markup could look like this:

```

model://reused-model

```

In order to determine if (nested) elements in the override section should be appended or actually override existing elements of the included model each tag (the tag name as well as its attributes) could be checked for equality. The override tag itself isn't necessary. The overriding elements could be direct children of `include`.

No files, tests, or entry points are named in the issue, so begin by locating the include-model parsing and model-element merge logic. Done should allow arbitrary valid model elements, including nested visual material overrides, while defining whether matching elements are replaced or appended.

Written by the indexing model from the issue text.