add_primitives add-on updated for Godot3

If someone is interested, I have updated old Godot2 module for generating simple meshes to Godot3: https://github.com/ppiecuch/godot-addons/tree/master/add_primitives

Not sure if everything works - it is quite large module, but at least most of the geom. primitives (Box, Capsule, Circle, Cone, Cylinder, Plane, Solid, Sphere, Torus, Tube, Wedge, Stair, Arch, C Box, Disc, Ellipse, Ellipsoid, L Box, Pyramid, Torus Knot) seems to be ok.