Added Primitives class

[PaperPrototype]

// create a ground cube with physics and colliders
var groundGo = Primitives.PhysicsCube("Ground", new Double3(20, 1, 200), isStatic: true);
scene.Add(groundGo);

// you can also create non physics primitives
var regularCube = Primitives.Cube("My cube", Double.One);
scene.Add(regularCube);

// provide your own material
var mat = new Material(Shader.LoadDefault(ShaderDefaults.Standard);
scene.Add(Primitives.Cube("My cube 2", Double.One, mat);

// provide your own mesh
var mesh = Mesh.CreateCube(Double.One);
scene.Add(Primitives.Cube("My cube 3", Double.One, mat, mesh);

Also automatically uses a cached mesh/material if you don't provide one :)