Cleanup models

[srmainwaring]

Model housekeeping: remove unused models and utilise #6 to split the models for easier composition.

Completes #34, except the regeneration of thumbnails.

Detail

• Remove the cmac world - the StaticMapPlugin is not available in Gazebo Sim.
• Rename world files and use the .sdf suffix.
• Update the runway model - reduce the gap between the airstrip and ground as upstream z-order has been fixed.
• Update the zephyr runway world - remove commented out pose.
• Update the iris warehouse world - remove models that do not load because of incorrect server details.
• Split the zephyr model into a base model and plugin model with ardupilot integration.
• Set the zephyr runway model to run with RTF > 1.
• Update README - describe how to run the zephyr model.