Incorrect URDF to SDF parsing materials

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/86)) by Allison Thackston (Bitbucket: [athackst](https://bitbucket.org/%7B28ae182d-47f4-4357-8024-ea9ef84e3559%7D/)).**

The original report had attachments: [test_urdf.sdf](https://osrf-migration.github.io/sdformat-gh-pages/data/repositories/osrf/sdformat/issues/86/attachments/test_urdf.sdf), [test_urdf.urdf](https://osrf-migration.github.io/sdformat-gh-pages/data/repositories/osrf/sdformat/issues/86/attachments/test_urdf.urdf), [test_urdf.xacro](https://osrf-migration.github.io/sdformat-gh-pages/data/repositories/osrf/sdformat/issues/86/attachments/test_urdf.xacro)

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Materials in URDF are added multiple times if the reference name is a subset of another name.

Example: link_slot1 assigned to color grey
link_slot1_switch assigned to color black has colors grey and black in sdf
link_slot1_switch_cover assigned to color red has colors grey, black and red in sdf

Start by comparing the attached test_urdf.urdf and test_urdf.xacro with the resulting test_urdf.sdf, focusing on material references whose names contain one another. Trace the URDF-to-SDF material conversion entry point and verify that each link receives only its intended material color; add a regression test if the existing test layout is identified.

Written by the indexing model from the issue text.