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:

https://github.com/robotology/gym-ignition/blob/50caccfa1b5ddb617ec0b1aea98bfec72c8fbadf/cpp/scenario/gazebo/include/scenario/gazebo/World.h#L135-L137

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