Support large DEM tiling for large outdoor terrain simulation

[Ryanf55]

Desired behavior

I want to simulate flight of an outdoor robot over a large DEM with collision enabled. The entire DEM dataset is too big for Gazebo to render, so it needs to be "tiled".

A nice-to-have would be ability to overlay real satellite imagery on the same terrain

Metrics

• I want to fly a standard fixed wing aircraft a distance of 100km in world coordinates.
• Traditional terrain data is spaced every 30 meters.

Alternatives considered

• Cut the terrain down to a small area and limit the flight area.

Implementation suggestion

• I have been recommended the levels, but could use help creating a more detailed design.

Additional context

See ROS-Aerial/community#65

This is a follow up to #596