Meshdata update

Some modification in meshdata structure and an example with qt backend to generate, cone, cylinder, sphere and arrow


For these arguments I suggest the following API:
```
cone_radius : float
    Radius of the cone base. If None, then this defaults to 2x the cylinder radius.
cone_length : float
    Length of the cone. If None, then this defaults to 1/3 of the cylinder length.
```