Insert a model in Gazebo with fixed pose

[diegoferigo]

In Gazebo Classic we could fix a model to the world by using an imported model from URI and a scoped joint (example icub_fixed_no_hands.sdf). I didn't manage to make this solution work with Ignition Gazebo. Maybe there's no support of scoped links / joints outside the original <model> element.

Since now we have the support of adding and removing model during runtime (#50), maybe we could set the base type (floating / fixed) directly in the ECM by adding a fixed joint entity during model creation.

If this works, we should add a bool fixed argument in one of the two following spots:

gym-ignition/ignition/include/gympp/gazebo/GazeboWrapper.h

Lines 87 to 88 in fef4b5c

	bool insertModel(const gympp::gazebo::ModelInitData& modelData,
	const gympp::gazebo::PluginData& pluginData = {});

gym-ignition/ignition/include/gympp/gazebo/GazeboWrapper.h

Lines 40 to 45 in fef4b5c

	struct gympp::gazebo::ModelInitData
	{
	std::string sdfString;
	std::string modelName = "";
	std::array<double, 3> position = {0, 0, 0};
	std::array<double, 4> orientation = {1, 0, 0, 0};

[diegoferigo]

Closed via #96