[WIP] Digital Elevation Model (DEM) tutorial

[jennuine]

🎉 New feature

Summary

Tutorial on preparing DEMs and using them in Gazebo.

Test it

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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