allow multiple <inertial> blocks in a single Link element

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/95)) by John Hsu (Bitbucket: [hsu](https://bitbucket.org/%7B0a186eae-abf0-4514-a951-23db5eccc286%7D/), GitHub: [hsu](https://github.com/hsu)).**

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This will make modeling slightly easier for end users. Currently, if a rigid body is composed of multiple simple geometries, user has to either:

1. compute combined inertia by hand, or
1. attach multiple links together with fixed joints

Allowing multiple `` blocks in a single link will leverage `gazebo::Inertia` class to compute combined inertia.