the XML file, including the path to the 3D file.
This change obsoletes the existing /sim/model/*-offset-deg and
/sim/model/*-offset-m properties in the main property tree, and
replaces them with /offsets/*-deg and /offsets/*-m in the model
property file. The /path property in the model XML file is relative
to the XML file's location rather than FG_ROOT.