Missing is/asPrismaticJoint in bindings

[Nicogene]

It is possible to get the handle of the RevoluteJoint and FixedJoint using the (python) bindings thanks to

idyntree/bindings/joints.i

Lines 15 to 41 in ab93364

	bool isRevoluteJoint() const
	{
	const iDynTree::RevoluteJoint * p =
	dynamic_cast<const iDynTree::RevoluteJoint*>($self);
	return (p != 0);
	}
	bool isFixedJoint() const
	{
	const iDynTree::FixedJoint * p =
	dynamic_cast<const iDynTree::FixedJoint*>($self);
	return (p != 0);
	}
	iDynTree::RevoluteJoint * asRevoluteJoint()
	{
	iDynTree::RevoluteJoint * p =
	static_cast<iDynTree::RevoluteJoint*>($self);
	return p;
	}
	iDynTree::FixedJoint * asFixedJoint()
	{
	iDynTree::FixedJoint * p =
	static_cast<iDynTree::FixedJoint*>($self);
	return p;
	}

It would be nice if these can be extended to other types of joints, in particular I would be like to have is/asPrismaticJoint

[traversaro]

It should be sufficient to add the code similar to the revolute one for the other types, feel free to open a PR, thanks!