Allow a subset of `<geometry>` instead of `<inertia>`

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/176)) by Shane Loretz (Bitbucket: [Shane Loretz](https://bitbucket.org/%7B656e3311-aad9-45a1-aaf7-b0ee0e84b287%7D/), GitHub: [sloretz](https://github.com/sloretz)).**

----------------------------------------
# Issue

The `` element requires an inertial matrix ``. When creating models I often find myself plugging the geometry of the `` into one of the formulas on [this wikipedia page](https://en.wikipedia.org/wiki/List_of_moments_of_inertia), which can be prone to errors as I often end up changing the geometry.

# Proposal

The proposal is to allow a subset of the shapes in `` (box, cylinder, sphere) as an alternative to ``. The inertia matrix values would be calculated by assuming uniform density.

```xml

0.5




1.0

0.5

```

# Related

Issue #95 combined with this feature would make it much easier to specify the inertia on a link with multiple ``.

Start by reviewing the proposed XML structure for , , and the box, cylinder, and sphere shapes, along with the linked moment-of-inertia formulas. Done means SDFormat accepts these shapes as an alternative to and calculates the matrix assuming uniform density, while preserving the existing mass requirement.

Written by the indexing model from the issue text.