Write a <geometry> once and reuse it

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/173)) by Shane Loretz (Bitbucket: [Shane Loretz](https://bitbucket.org/%7B656e3311-aad9-45a1-aaf7-b0ee0e84b287%7D/), GitHub: [sloretz](https://github.com/sloretz)).**

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# Issue

It feels tedious to type or copy/paste `` when multiple collisions/visuals use the same geometry. Making changes becomes prone to copy/paste errors when a lot of places have the same information.

# Proposal

* Add an optional attribute `name` to ``
* Add a child tag `` that allows referencing a geometry by a scoped name

```xml

1 1 1




model://my_car/some_link/its_visual/some_geom

```

# Alternatives

An alternative to this proposal is to have a tag that contains geometries that can be referenced. This is similar to Collada's `` ([Chapter 5 page 65](https://www.khronos.org/files/collada_spec_1_4.pdf))

Enhancement #75 could be an alternative to this proposal by allowing `` to include ``.

No source files or tests are named. Start by locating the existing parser and any name or URI resolution logic, then evaluate the proposed named geometry and scoped reference against the library's current format rules. Done means repeated geometry can be defined once and referenced by scoped name, with the proposal's XML example covered by appropriate validation.

Written by the indexing model from the issue text.