Add temperature to links and/or models.

A thermal camera requires temperature properties to be associated with models. This is currently achieved in Ignition Gazebo through a plugin.

It seems pretty straightforward to add a `` tag to SDF. Specification of a position for the heat source would be nice.

Start by tracing how the SDF parser and description files represent links and models, then review how Ignition Gazebo currently supplies temperature properties through a plugin. Define what the new temperature element and heat-source position must represent, and consider the parser and description-file changes needed for complete support.

Written by the indexing model from the issue text.