Including an SDFormat model file containing multiple models is ambiguous

[azeey]

According to the spec, an SDFormat model file can contain multiple models, i.e one can have:

<?xml version='1.0'?>
<sdf version='1.7'>
 <model name='model_1'>
    ...
 </model>
 <model name='model_2'>
    ...
 </model>
</sdf>

However, it is not clear what should happen when such a model file is used with the //include tag. Are both models included or just one? If both are included, which model do the overriding elements in //include (eg. //include/name, //include/pose) refer to?

[azeey]

Per our discussion with @EricCousineau-TRI, @scpeters, and @sammy-tri, SDFormat files where the contents are //sdf/model, //sdf/actor, or //sdf/light should only contain one of those elements. The rationale is that these files can be included with //include while overriding some of their properties. Having multiple elements in the file would create confusion as to which element is being included.