Support SDF's relative_to when inserting models

[diegoferigo]

The new frame semantics feature of SDF allows specifying a relative_to attribute in the pose element. It could be very useful to insert models programmatically in a location relative to other models:

gym-ignition/cpp/scenario/gazebo/include/scenario/gazebo/World.h

Lines 135 to 137 in 50caccf

	bool insertModel(const std::string& modelFile,
	const core::Pose& pose = core::Pose::Identity(),
	const std::string& overrideModelName = {});

A possible implementation could extent core::Pose (in a backward-compatible way) so that the World::insertModel* methods do not need any new argument.